Populate request headers in shouldRateLimit rpc response

[yusofg2]

Problem

The service that uses global rate limits does not have visibility into rate limiting buckets. Some services require the knowledge of rate limit quota usage.

Use-case

A service sets up rate limits in shadow mode and needs to serve requests that are over limit differently than regularly admitted requests.

Solution

The shouldRateLimit API definition already includes request_headers_to_add in the response to be forwarded to the upstream service but the ratelimit service does not populate the field. We implement population of those headers very similar to the existing response_headers_to_add.
The new configuration parameters are listed in README.md while the design specification and implementation plan are included under docs/superpowers.
In the implementation, a minor bug related to config reload when response headers are disabled was found and fixed in commit 85fb09a.

Testing

For local integration testing:

1. RLS is configured with LIMIT_REQUEST_HEADERS_ENABLED enabled.
2. envoy-mock service that acts as the upstream is configured to log the rate limit request headers.
3. Verified requests sent by integration tests are injected with the rate limit request headers by the envoy sidecar and received and logged by envoy-mock, e.g. run

docker logs integration-test-envoy-mock-1 2>&1 | grep "mock-upstream"

Here is a sample output:

     [mock-upstream] method=GET path=/multiservice-tokenquota RateLimit-Limit=1 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/multiservice-tokenquota RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/multiservice-tokenquota RateLimit-Limit=1 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/quota RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9                                               
     [mock-upstream] method=GET path=/quota RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/quota RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9                                                   
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=3 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=2 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=3 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=2 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fiveheader RateLimit-Limit=4 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/twoheader RateLimit-Limit=1 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/fourheader RateLimit-Limit=2 RateLimit-Remaining=0 RateLimit-Reset=9
     [mock-upstream] method=GET path=/twoheader RateLimit-Limit=3 RateLimit-Remaining=2 RateLimit-Reset=9
     [mock-upstream] method=GET path=/tokenquota RateLimit-Limit=2 RateLimit-Remaining=1 RateLimit-Reset=9
     [mock-upstream] method=GET path=/tokenquota RateLimit-Limit=2 RateLimit-Remaining=0 RateLimit-Reset=9

TBD

1. Include Claude superpowers artifacts in the open source PR?
2. Individual commits are not squashed before to show the development and review process.