Summary:
Postgres maintains a BackendStatusArray structure in shared memory. We read from this structure to gather data for /rpcz.
Currently the response is missing the query id and leader pid fields so this revision adds those fields.
Jira: DB-14851

Test Plan:
Manually verified the changes compile successfully and the new fields show up in /rpcz response.
Tested locally using yugabyted and yb sample apps.

Reviewers: telgersma

Reviewed By: telgersma

Subscribers: smishra, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41129