This PR adds the view `grpc.io/client/api_latency` for GCP Observability
which aims to collect the end-to-end time taken by a call.
Changes made to support this -
1) A global interceptor factory registration is created for stats
plugins.
2) OpenCensus plugin now provides a new interceptor that's responsible
for collecting the new latency.
3) Gcp Observability registers this plugin.
4) A new OpenCensus measurement and view is created for api latency.
Note that this is internal as of now, since it's not clear if it should
be exposed as public experimental API. Leaving that decision for the
future.
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