[v26.1.x] rpc: derive requests_pending gauge from _correlations

Backport https://github.com/redpanda-data/redpanda/pull/30264 to branch v26.1.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked. Here are the commands to execute: ``` git checkout v26.1.x git checkout -b manual-backport-30264-v26.1.x-396 git cherry-pick -x 29cd963cf4 git push origin manual-backport-30264-v26.1.x-396 gh pr create \ --title "[v26.1.x] rpc: derive requests_pending gauge from _correlations" \ --base "v26.1.x" \ --label "kind/backport" \ --head "manual-backport-30264-v26.1.x-396" \ --draft \ --repo "redpanda-data/redpanda" \ --milestone "v26.1.x-next" \ --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/30264 "