Security Token Service (STS) to fetch grpc call credentials

[mandarjog]

OC tracer and OC metrics should support token fetching using STS service.
https://tools.ietf.org/html/draft-ietf-oauth-token-exchange-16

Stackdriver metrics and tracing API both need STS to support GKE workload identity.

Example Envoy PR adds this to Envoy.

1. Update stackdriver stats exporter
2. Update stackdriver trace exporter

Use json config which can be passed through from Envoy.

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