Removing redundant constant. (#4588)
It looks like it was moved to `google.api_core.gapic_v1.client_info`
and during the move the delete was forgotten.
diff --git a/google/api_core/gapic_v1/method.py b/google/api_core/gapic_v1/method.py
index 0e84a84..50d4356 100644
--- a/google/api_core/gapic_v1/method.py
+++ b/google/api_core/gapic_v1/method.py
@@ -23,7 +23,6 @@
from google.api_core import timeout
from google.api_core.gapic_v1 import client_info
-METRICS_METADATA_KEY = 'x-goog-api-client'
USE_DEFAULT_METADATA = object()
DEFAULT = object()
"""Sentinel value indicating that a retry or timeout argument was unspecified,