Refactor 'client_info' support. (#7849)

* Add 'user_agent' and 'extra_headers' properties to 'Connection'.

   Deprecate the 'USER_AGENT' and '_EXTRA_HEADERS' class-level attributes.

* Add 'client_info' parameter to 'Connection' ctor.

* Implement 'Connection.user_agent' via its '_client_info'.

* Ensure 'X-Goog-API-Client' header is always passed.

* Create/use non-GAPIC-specific 'ClientInfo' class FBO HTTP/JSON clients.

  Derive the existing GAPIC class from it.
diff --git a/google/api_core/client_info.py b/google/api_core/client_info.py
new file mode 100644
index 0000000..b196b7a
--- /dev/null
+++ b/google/api_core/client_info.py
@@ -0,0 +1,96 @@
+# Copyright 2017 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Helpers for providing client information.
+
+Client information is used to send information about the calling client,
+such as the library and Python version, to API services.
+"""
+
+import platform
+
+import pkg_resources
+
+_PY_VERSION = platform.python_version()
+_API_CORE_VERSION = pkg_resources.get_distribution("google-api-core").version
+
+try:
+    _GRPC_VERSION = pkg_resources.get_distribution("grpcio").version
+except pkg_resources.DistributionNotFound:  # pragma: NO COVER
+    _GRPC_VERSION = None
+
+
+class ClientInfo(object):
+    """Client information used to generate a user-agent for API calls.
+
+    This user-agent information is sent along with API calls to allow the
+    receiving service to do analytics on which versions of Python and Google
+    libraries are being used.
+
+    Args:
+        python_version (str): The Python interpreter version, for example,
+            ``'2.7.13'``.
+        grpc_version (Optional[str]): The gRPC library version.
+        api_core_version (str): The google-api-core library version.
+        gapic_version (Optional[str]): The sversion of gapic-generated client
+            library, if the library was generated by gapic.
+        client_library_version (Optional[str]): The version of the client
+            library, generally used if the client library was not generated
+            by gapic or if additional functionality was built on top of
+            a gapic client library.
+        user_agent (Optional[str]): Prefix to the user agent header. This is
+            used to supply information such as application name or partner tool.
+            Recommended format: ``application-or-tool-ID/major.minor.version``.
+    """
+
+    def __init__(
+        self,
+        python_version=_PY_VERSION,
+        grpc_version=_GRPC_VERSION,
+        api_core_version=_API_CORE_VERSION,
+        gapic_version=None,
+        client_library_version=None,
+        user_agent=None,
+    ):
+        self.python_version = python_version
+        self.grpc_version = grpc_version
+        self.api_core_version = api_core_version
+        self.gapic_version = gapic_version
+        self.client_library_version = client_library_version
+        self.user_agent = user_agent
+
+    def to_user_agent(self):
+        """Returns the user-agent string for this client info."""
+
+        # Note: the order here is important as the internal metrics system
+        # expects these items to be in specific locations.
+        ua = ""
+
+        if self.user_agent is not None:
+            ua += "{user_agent} "
+
+        ua += "gl-python/{python_version} "
+
+        if self.grpc_version is not None:
+            ua += "grpc/{grpc_version} "
+
+        ua += "gax/{api_core_version} "
+
+        if self.gapic_version is not None:
+            ua += "gapic/{gapic_version} "
+
+        if self.client_library_version is not None:
+            ua += "gccl/{client_library_version} "
+
+        return ua.format(**self.__dict__).strip()
diff --git a/google/api_core/gapic_v1/client_info.py b/google/api_core/gapic_v1/client_info.py
index 069e019..bdc2ce4 100644
--- a/google/api_core/gapic_v1/client_info.py
+++ b/google/api_core/gapic_v1/client_info.py
@@ -18,22 +18,13 @@
 such as the library and Python version, to API services.
 """
 
-import platform
+from google.api_core import client_info
 
-import pkg_resources
-
-_PY_VERSION = platform.python_version()
-_API_CORE_VERSION = pkg_resources.get_distribution("google-api-core").version
-
-try:
-    _GRPC_VERSION = pkg_resources.get_distribution("grpcio").version
-except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-    _GRPC_VERSION = None
 
 METRICS_METADATA_KEY = "x-goog-api-client"
 
 
-class ClientInfo(object):
+class ClientInfo(client_info.ClientInfo):
     """Client information used to generate a user-agent for API calls.
 
     This user-agent information is sent along with API calls to allow the
@@ -56,47 +47,6 @@
             Recommended format: ``application-or-tool-ID/major.minor.version``.
     """
 
-    def __init__(
-        self,
-        python_version=_PY_VERSION,
-        grpc_version=_GRPC_VERSION,
-        api_core_version=_API_CORE_VERSION,
-        gapic_version=None,
-        client_library_version=None,
-        user_agent=None,
-    ):
-        self.python_version = python_version
-        self.grpc_version = grpc_version
-        self.api_core_version = api_core_version
-        self.gapic_version = gapic_version
-        self.client_library_version = client_library_version
-        self.user_agent = user_agent
-
-    def to_user_agent(self):
-        """Returns the user-agent string for this client info."""
-
-        # Note: the order here is important as the internal metrics system
-        # expects these items to be in specific locations.
-        ua = ""
-
-        if self.user_agent is not None:
-            ua += "{user_agent} "
-
-        ua += "gl-python/{python_version} "
-
-        if self.grpc_version is not None:
-            ua += "grpc/{grpc_version} "
-
-        ua += "gax/{api_core_version} "
-
-        if self.gapic_version is not None:
-            ua += "gapic/{gapic_version} "
-
-        if self.client_library_version is not None:
-            ua += "gccl/{client_library_version} "
-
-        return ua.format(**self.__dict__).strip()
-
     def to_grpc_metadata(self):
         """Returns the gRPC metadata for this client info."""
         return (METRICS_METADATA_KEY, self.to_user_agent())
diff --git a/tests/unit/gapic/test_client_info.py b/tests/unit/gapic/test_client_info.py
index 0cca479..64080ff 100644
--- a/tests/unit/gapic/test_client_info.py
+++ b/tests/unit/gapic/test_client_info.py
@@ -16,59 +16,6 @@
 from google.api_core.gapic_v1 import client_info
 
 
-def test_constructor_defaults():
-    info = client_info.ClientInfo()
-
-    assert info.python_version is not None
-    assert info.grpc_version is not None
-    assert info.api_core_version is not None
-    assert info.gapic_version is None
-    assert info.client_library_version is None
-
-
-def test_constructor_options():
-    info = client_info.ClientInfo(
-        python_version="1",
-        grpc_version="2",
-        api_core_version="3",
-        gapic_version="4",
-        client_library_version="5",
-        user_agent="6"
-    )
-
-    assert info.python_version == "1"
-    assert info.grpc_version == "2"
-    assert info.api_core_version == "3"
-    assert info.gapic_version == "4"
-    assert info.client_library_version == "5"
-    assert info.user_agent == "6"
-
-
-def test_to_user_agent_minimal():
-    info = client_info.ClientInfo(
-        python_version="1", api_core_version="2", grpc_version=None
-    )
-
-    user_agent = info.to_user_agent()
-
-    assert user_agent == "gl-python/1 gax/2"
-
-
-def test_to_user_agent_full():
-    info = client_info.ClientInfo(
-        python_version="1",
-        grpc_version="2",
-        api_core_version="3",
-        gapic_version="4",
-        client_library_version="5",
-        user_agent="app-name/1.0",
-    )
-
-    user_agent = info.to_user_agent()
-
-    assert user_agent == "app-name/1.0 gl-python/1 grpc/2 gax/3 gapic/4 gccl/5"
-
-
 def test_to_grpc_metadata():
     info = client_info.ClientInfo()
 
diff --git a/tests/unit/test_client_info.py b/tests/unit/test_client_info.py
new file mode 100644
index 0000000..0eb17c5
--- /dev/null
+++ b/tests/unit/test_client_info.py
@@ -0,0 +1,69 @@
+# Copyright 2017 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+from google.api_core import client_info
+
+
+def test_constructor_defaults():
+    info = client_info.ClientInfo()
+
+    assert info.python_version is not None
+    assert info.grpc_version is not None
+    assert info.api_core_version is not None
+    assert info.gapic_version is None
+    assert info.client_library_version is None
+
+
+def test_constructor_options():
+    info = client_info.ClientInfo(
+        python_version="1",
+        grpc_version="2",
+        api_core_version="3",
+        gapic_version="4",
+        client_library_version="5",
+        user_agent="6"
+    )
+
+    assert info.python_version == "1"
+    assert info.grpc_version == "2"
+    assert info.api_core_version == "3"
+    assert info.gapic_version == "4"
+    assert info.client_library_version == "5"
+    assert info.user_agent == "6"
+
+
+def test_to_user_agent_minimal():
+    info = client_info.ClientInfo(
+        python_version="1", api_core_version="2", grpc_version=None
+    )
+
+    user_agent = info.to_user_agent()
+
+    assert user_agent == "gl-python/1 gax/2"
+
+
+def test_to_user_agent_full():
+    info = client_info.ClientInfo(
+        python_version="1",
+        grpc_version="2",
+        api_core_version="3",
+        gapic_version="4",
+        client_library_version="5",
+        user_agent="app-name/1.0",
+    )
+
+    user_agent = info.to_user_agent()
+
+    assert user_agent == "app-name/1.0 gl-python/1 grpc/2 gax/3 gapic/4 gccl/5"