feat: Move the metrics handler fixture (#1570)

Summary:

This PR simply moves the metrics handler fixture used for tests from the `test` folder to the `test-common` folder. In the next PR the metrics handler fixture will be used for both tests and system tests and when system tests import a file from the test folder and we run `npm run system-test` it runs both the test and the system test suite and we don't want that so right now we are moving the fixture to the `test-common` folder.

Next steps:

The diagram below shows how the fixture is going to be used in the next PR in system tests.

![With fixture - 2nd plumbing](https://github.com/user-attachments/assets/7eedf01b-8a30-4fbc-af6e-f1e17eb4c4f3)

Author: danieljbruce

test/metrics-collector/metrics-handler-fixture.ts test-common/metrics-handler-fixture.ts

@@ -22,7 +22,7 @@ import {
   StreamingState,
 } from '../../src/client-side-metrics/client-side-metrics-attributes';
 import {grpc} from 'google-gax';
-import {expectedRequestsHandled} from './metrics-handler-fixture';
+import {expectedRequestsHandled} from '../../test-common/metrics-handler-fixture';
 import * as gax from 'google-gax';
 const root = gax.protobuf.loadSync(
   './protos/google/bigtable/v2/response_params.proto'