pyroscope.write: add optional http client trace log

[korniltsev-grafanista]

PR Description

1. Add an optional http client trace log.
2. Unify retry logic into a common forward function
3. Unify useragent usage

Example trace log.

test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.599851759+07:00 msg=GetConn hostPort=127.0.0.1:45201
test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.599859193+07:00 msg=GotConn Reused=true WasIdle=true IdleTime=231.789µs Conn="unsupported value type"
test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.599866547+07:00 msg=WroteHeaders
test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.599890362+07:00 msg=WroteRequest Err=null
test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.59996306+07:00 msg=GotFirstResponseByte
test=Test_Write_HttpClientTrace/error_only_tracing_error ts=06:33:58.600 level=debug trace_id=00000000000000000000000000000000 op=ingest tt=2025-09-08T13:33:58.599969232+07:00 msg=PutIdleConn err=null

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated