Increase unit testing coverage in the metrics package

Increase unit testing coverage in the [metrics package](https://github.com/apache/beam/tree/release-2.36.0/sdks/go/pkg/beam/core/metrics)

We want code coverage at or above 50%, it is currently at 43.5%.

Imported from Jira [BEAM-13900](https://issues.apache.org/jira/browse/BEAM-13900). Original Jira may contain additional context.
Reported by: jrmccluskey.
Subtask of issue #21383