Backport of [API Gateway] Fix rate limiting for API gateways into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #17631 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

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Description

We noticed when we were running the Consul Kubernetes conformance tests that we were getting strange rate-limiting errors in Consul. I tracked this down to the fact that the rate limiting code internally is using the service usage metrics code in our state store and noticed that API gateways haven't been added there, so the downstream places where we count and rate limit based on the number of connect-enabled services would never include API gateways in the count. I'm assuming that the reason that we've not noticed up until this point is that:

1. there are some rounding issues that make it so we generally don't have a problem with a single gateway connecting up to Consul as in most of our behavioral acceptance/integration tests
2. there are enough places where we have race conditions between services racing to consume the allowed xDS connections that we generally had our gateways getting one of the said connections.

Testing & Reproduction steps

To reproduce this bug you can run conformance tests against the current Consul Kubernetes code or just register a bunch of services and API Gateways and then watch the Consul rate limiter start throwing errors.

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• fb2f3b6 - 178abb8 - 77b3998 - a245b32

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR