[fix] bound what one anonymous push request can consume

[Duansg]

What's changed?

Add three configurable caps to the push gateway: auto-created monitors, request body size, and sample count.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[zqr10159]

There is still a concurrency hole in the auto-created-monitor cap. containsKey/size is checked before computeIfAbsent; multiple new job/instance keys can all observe capacity and then insert, exceeding the configured maximum. The current tests exercise sequential calls only. Please make reservation atomic (for example with a semaphore/explicit reservation or a truly bounded cache) and add a concurrent burst test that proves the map never exceeds the cap. The request-body and sample-count limits are still useful independently.