tp: move metrics descriptor generation to build time

This is necessary because generating the descriptor at check-in time was
leading to conflicts in G3 where the third_party/perfetto prefix is
present for extensions defined in Google3 but not the builtin
descriptors.

This CL ensures that wherever we build, we get a consistent generation
of the builtin metric descriptor.

Change-Id: I94be63dae7960edc8aabac8f47686a82daa4796e
diff --git a/BUILD.extras b/BUILD.extras
index d7c711b..83a17f0 100644
--- a/BUILD.extras
+++ b/BUILD.extras
@@ -22,6 +22,15 @@
     python_version = "PY3",
 )
 
+perfetto_py_binary(
+    name = "gen_cc_proto_descriptor_py",
+    srcs = [
+        "tools/gen_cc_proto_descriptor.py",
+    ],
+    main = "tools/gen_cc_proto_descriptor.py",
+    python_version = "PY3",
+)
+
 perfetto_java_proto_library(
     name = "protos_perfetto_metrics_java",
     deps = [