ref: add metric when canonical key falls back (#70385)

want to make sure this isn't happening before I clean up all of this --
I'm told this is handled by relay now

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Author: asottile-sentry

src/sentry/utils/canonical.py

@@ -43,7 +43,14 @@
 
 
 def get_canonical_name(key: str) -> str:
-    return CANONICAL_KEY_MAPPING.get(key, (key,))[0]
+    rv = CANONICAL_KEY_MAPPING.get(key)
+    if rv is None:
+        return key
+    else:
+        from sentry.utils import metrics
+
+        metrics.incr("canonical-legacy-key")
+        return rv[0]
 
 
 class CanonicalKeyView(Mapping[str, V]):