Match the host header case-insensitively in UpstreamGrpcSender

[thswlsqls]

Fixes #8956

Description

• UpstreamGrpcSender.send() used headers.get("host") to find the authority override, so a header added as Host or HOST was sent as plain metadata and :authority stayed the endpoint address.
• The check now uses equalsIgnoreCase("host") inside the existing header loop. Lowercase host behaves as before.
• This matches the default OkHttp sender, which Allow overriding OTLP gRPC authority. #4514 aimed to mirror. HTTP field names are case-insensitive (RFC 9110 §5.1).

Testing done

• Added AbstractGrpcTelemetryExporterTest#overrideHostMixedCase: overrideHost with addHeader("Host", "opentelemetry"). It runs for span, metric and log exporters on every gRPC sender suite.
• Without the main change, ./gradlew :exporters:otlp:all:testGrpcNetty --tests '*overrideHostMixedCase*' fails 3/3 with expected: "opentelemetry" but was: "127.0.0.1:<port>".
• With the change, all pass: testGrpcNetty, testGrpcNettyShaded, testGrpcOkhttp (198 tests each, 27 skipped), testOkHttpVersionLATEST (345 tests, 4 skipped).
• :exporters:sender:grpc-managed-channel:check and :exporters:otlp:testing-internal:check pass. No public API change.

[opentelemetry-pr-dashboard]

Pull request dashboard status

Waiting on reviewers · refreshed 2026-10-18 13:28 UTC

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.41%. Comparing base (b32298a) to head (003cc16).

Files with missing lines	Patch %	Lines
...pc/managedchannel/internal/UpstreamGrpcSender.java	50.00%	0 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8957   +/-   ##
=========================================
  Coverage     90.40%   90.41%           
  Complexity    10898    10898           
=========================================
  Files          1015     1015           
  Lines         29843    29842    -1     
  Branches       3896     3896           
=========================================
  Hits          26981    26981           
+ Misses         1910     1909    -1     
  Partials        952      952           

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