Add Envoy upstream_rq_completed cluster metrics

[csssuf]

What does this PR do?

Add the following metrics to the Envoy integration:

• cluster.upstream_rq_completed
• cluster.canary.upstream_rq_completed
• cluster.internal.upstream_rq_completed
• cluster.external.upstream_rq_completed

Motivation

These metrics are published by Envoy but not exposed by Datadog, but are useful for calculating request success rates.

Additional Notes

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[therve]

Thanks for your contribution!