Merge pull request #49678 from smarterclayton/429_metric

Automatic merge from submit-queue

Timeout and Max-in-flight don't report non-resource URLs correctly.

Unify error reporting for 429 and 504 to be correct for timeout and max in flight and eviction. Add better messages to eviction (removing a todo). Return the correct body content for timeouts (reason and code should be correct).

This potentially increases cardinality of 429, but because non-api urls may be under the max-inflight budget we need to report them somewhere (if something breaks and starts fetching API versions endlessly).

```release-note
The 504 timeout error was returning a JSON error body that indicated it was a 500.  The body contents now correctly report a 500 error.
```

api/openapi-spec/swagger.json

@@ -61376,7 +61376,7 @@
       "type": "string"
      },
      "retryAfterSeconds": {
-      "description": "If specified, the time in seconds before the operation should be retried.",
+      "description": "If specified, the time in seconds before the operation should be retried. Some errors may indicate the client must take an alternate action - for those errors this field may indicate how long to wait before taking the alternate action.",
       "type": "integer",
       "format": "int32"
      },