Problem

When sending an x-request-id header through SpiceDB's HTTP gateway, the request ID is processed internally but not returned in the HTTP response headers, breaking standard HTTP patterns.

Current Behaviour

Response headers contain: Grpc-Metadata-Io.spicedb.respmeta.requestid: test-123, yet, response headers are still missing: x-request-id: test-123

Investigation

SpiceDB's request ID middleware uses requestmeta.RequestIDKey and responsemeta.RequestID from this lib.
In commit 34b8159 we added RequestIDKey using the standard x-request-id header. However, responsemeta.RequestID still uses the old custom key io.spicedb.respmeta.requestid.
This means requests use the std header but responses use a custom one. Hence, we would have to add a custom header translation in SpiceDB's grpc gateway to deal with this. Instead, let's update responsemeta.RequestID to x-request-id.

Note: This will be a small breaking change for any code that hardcodes io.spicedb.respmeta.requestid instead of using the responsemeta.RequestID constant. Once we release a new version of authzed-go, we can open a PR in SpiceDB to include these changes.