[BUGFIX] Complete an OTLP HTTP request on every state its client can end on

[thc1006]

Fixes #4425.

ResponseHandler::OnEvent decides a request is over from a switch naming seven states. The client can end a transfer on three more, which the switch below it recognises and logs:

State	Reported as over	Logged
CreateFailed, ConnectFailed, SendFailed, SSLHandshakeFailed, TimedOut, NetworkError, Cancelled	yes	yes
Destroyed, ReadError, WriteError	no	yes

A request that ends on one of those three is described and then dropped. It never reaches Unbind, its result callback never runs and its session is never released, so ForceFlush and Shutdown wait out their own deadlines and the caller waits for a callback that is not coming.

The three states join the same switch. That is the whole change, seven lines in otlp_http_client.cc.

The bundled curl client does not produce any of them, so this is reached through the installed HttpClient interface by a client the user supplies. SessionState is part of that interface, and the header describes these three as a read error, a write error and a destroyed session, so an exporter is not in a position to decide which of them a client may use.

Exactly-once is unchanged. stopping_ is a compare exchange and the first writer wins, so one of these arriving after a response has already been reported still reports nothing.

Two cases in otlp_http_exporter_custom_client_test.cc cover it: each of the three delivered as the only terminal event, and each delivered after a response. Removing the three lines does not turn them red, it hangs the binary on a session that never settles, which is the shape the defect has for a user.

@lalitb noted on #4448 that this can go ahead independently of the transport design. It changes no curl code.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.73%. Comparing base (1c2b007) to head (f5e17e2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4453      +/-   ##
==========================================
+ Coverage   82.67%   82.73%   +0.06%     
==========================================
  Files         516      516              
  Lines       20197    20197              
==========================================
+ Hits        16696    16708      +12     
+ Misses       3501     3489      -12     

Files with missing lines	Coverage Δ
exporters/otlp/src/otlp_http_client.cc	73.48% <ø> (+2.36%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.