docs: Add OC stats example code

[c24t]

Fixes GoogleCloudPlatform/python-docs-samples#6514 🦕,

Moved from GoogleCloudPlatform/python-docs-samples#6515.

The last PR included this note:

Note that we need to do extra work to instrument gRPC to emit the stats we're collecting here. We may want to wait to merge this PR or include this in public docs until we make the required gRPC changes.

But to make this clearer: this code won't emit gRPC stats because gRPC-python itself doesn't support OpenCensus tracing. See https://opencensus.io/guides/grpc/.

The sample also uses measures and views from the OpenCensus gRPC Stats
specification. These ought to be moved somewhere like opencensus-ext-grpc.

@larkee I put these samples in a separate samples/opencensus/ dir because the other samples don't require OC, let me know if you want this arranged differently.

[snippet-bot]

Here is the summary of changes.

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• samples/opencensus/capture_grpc_metric.py:21, tag spanner_opencensus_capture_grpc_metric

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