Implement the StringSet metric type in Prism.

This may require regeneration of the Go Protocol buffers since the metric type is very new: See https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/model/PROTOBUF.md

A metric accumulator will need to be added for the type in prism:

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/prism/internal/jobservices/metrics.go#L357

And adding the type to the accumulator factory list:

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/prism/internal/jobservices/metrics.go#L168

Specifically with the right URN:

https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/org/apache/beam/model/pipeline/v1/metrics.proto#L576

This would allow the following Java Validates Runner Tests to Pass.

MetricsTest.testCommittedStringSetMetrics
MetricsTest.testAttemptedStringSetMetrics

It's out of scope for this issue to implement the StringSet metric in the Go SDK.