Add ocgrpc self-obs metrics in test fixtures

[aabmass]

• Enable the ocgrpc views in integration tests
• Some of the ocgrpc metrics like latency and byte sizes are not reproducible across test runs, so we normalize the values away but still check the type and label sets of these metrics. This will still capture histogram bucket boundaries and value types.
• Update the value field to a oneof with a slimmed down Histogram option.
• Added a metric type field to the fixture which should be roughly what would be sent to GCM for the metric.