Implement histogram user metric for python SDK

[hjtran]

Resolves #36267

Implement the Histogram metric for the python SDK.

I'm going to do the actual moving of all histogram related stuff from internal/metric.py to the public metric.py in a separate PR to make this PR easier to review.

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[hjtran]

failure test_enrichment_with_bigtable (apache_beam.examples.snippets.transforms.elementwise.enrichment_test.EnrichmentTest) looks unrelated

[hjtran]

assign set of reviewers

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[hjtran]

Hey @claudevdm , do you have time to review this? No worries if not, I can request another reviewer if so

[claudevdm]

R: @claudevdm

Sorry missed this, will take a look today or tomorrow.

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[hjtran]

Thanks for the review @claudevdm !