[BUG] Report an Elasticsearch async export's outcome exactly once

[thc1006]

Fixes #4338.

AsyncResponseHandler calls the exporter's result callback directly from OnResponse and from each terminal OnEvent state with no guard, and ReadError, WriteError and Destroyed fall through a default label and call nothing.

The HTTP client can deliver both a response and a terminal event for one request, so one export can report twice. It can also end on one of those three states and report nothing at all. The exporter counts one finished session per export, so the first overshoots that count for the life of the exporter, and the second leaves a flush waiting on a session that has already ended.

Every path goes through one CompleteOnce now, a compare exchange that reports at most once and keeps the first verdict. The switch lists every state with no default, so a state added upstream fails to compile rather than going uncounted, and the destructor reports a failure for a handler torn down without an outcome.

One thing comes with it that is not strictly the fix. The completion line said trace span(s) in the log exporter; it says log record(s) now. The cases below read that line to count outcomes, and the wording was wrong either way.

Nine cases in es_log_record_exporter_test.cc, driven by a fake HTTP client through the public constructor: each terminal ordering a real session can produce, a response and a teardown event in both orders, and the concurrent version of each. Removing the compare exchange turns six of the nine red.

Extracted from #4337, which is 1526 lines and closes two issues. What stays there is the ForceFlush deadline and watermark accounting for #4336, including four completion cases that verify this guard through the flush rather than through the log line, which need that accounting to work.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

❌ Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.73%. Comparing base (312acb6) to head (9d5224c).

Files with missing lines	Patch %	Lines
...orters/elasticsearch/src/es_log_record_exporter.cc	89.19%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4502      +/-   ##
==========================================
+ Coverage   83.47%   83.73%   +0.26%     
==========================================
  Files         522      522              
  Lines       20440    20453      +13     
==========================================
+ Hits        17061    17124      +63     
+ Misses       3379     3329      -50     

Files with missing lines	Coverage Δ
...orters/elasticsearch/src/es_log_record_exporter.cc	86.90% <89.19%> (+39.17%)	⬆️

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