Export string sets in monitoring infos.

[robertwb]

This ensures they'll be available on Dataflow Runner v2.

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[robertwb]

R: @Abacn

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[Abacn]

Checkout this PR, removed excludeCategory in commonRunnerV2ExcludeCategories and ran ./gradlew :runners:google-cloud-dataflow-java:validatesRunnerV2Test --tests *test*StringSetMetrics manually, still get the same AssertionError

org.apache.beam.sdk.metrics.MetricsTest$AttemptedMetricTests

java.lang.AssertionError: 
Expected: iterable with items [MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sources", step="MyStep1", attempted=<StringSetResult{stringSet=[gcs]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sinks", step="MyStep2", attempted=<StringSetResult{stringSet=[kafka, bq]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sideinputs", step="MyStep1", attempted=<StringSetResult{stringSet=[bigtable, spanner]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sideinputs", step="MyStep2", attempted=<StringSetResult{stringSet=[sql, bigtable]}>}] in any order
     but: no item matches: MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sources", step="MyStep1", attempted=<StringSetResult{stringSet=[gcs]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sinks", step="MyStep2", attempted=<StringSetResult{stringSet=[kafka, bq]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sideinputs", step="MyStep1", attempted=<StringSetResult{stringSet=[bigtable, spanner]}>}, MetricResult{inNamespace="org.apache.beam.sdk.metrics.MetricsTest", name="sideinputs", step="MyStep2", attempted=<StringSetResult{stringSet=[sql, bigtable]}>} in []
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:6)
	at org.apache.beam.sdk.metrics.MetricsTest.assertStringSetMetrics(MetricsTest.java:449)

is this expected? (there is still some gap to make this metrics available in PipelineResult

[robertwb]

is this expected? (there is still some gap to make this metrics available in PipelineResult

Yes, there's still work to plumb the results back on the Dataflow service end.

[Abacn]

thanks

[Abacn]

minor: gaugeCell -> stringSetSell

[robertwb]

Done.