Deprecate TextMapGetter keys method

[arnabnandy7]

Description

Deprecates TextMapGetter.keys() and gives it a default empty implementation so TextMapGetter can be used as a functional interface.

This lets callers implement TextMapGetter with a lambda when only get() behavior is needed, while preserving existing carrier-key enumeration behavior for implementations that override keys().

Changes

• Marked TextMapGetter as @FunctionalInterface.
• Deprecated TextMapGetter.keys().
• Added a default keys() implementation returning an empty iterable.
• Added a regression test covering lambda-based TextMapGetter implementation.
• Suppressed intentional deprecated keys() usage in deprecated Jaeger and OT trace baggage extraction paths.
• Updated the opentelemetry-context API diff.

Testing

• ./gradlew :context:test --tests io.opentelemetry.context.propagation.TextMapGetterTest
• ./gradlew :extensions:trace-propagators:compileJava
• ./gradlew :context:jApiCmp
• ./gradlew :context:spotlessApply :extensions:trace-propagators:spotlessApply
• git diff --check

Fixes #8470 and dependent on spec:5177

[otelbot]

⚠️ API changes detected — additional maintainer review required

@jack-berg @jkwatson

This PR modifies the public API surface area of the following module(s):

• opentelemetry-context

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.94%. Comparing base (77eb68d) to head (e761257).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8531   +/-   ##
=========================================
  Coverage     90.94%   90.94%           
- Complexity    10210    10211    +1     
=========================================
  Files          1013     1013           
  Lines         27175    27176    +1     
  Branches       3184     3184           
=========================================
+ Hits          24714    24715    +1     
  Misses         1734     1734           
  Partials        727      727           

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[jack-berg]

Added blocked:spec label. We can revisit if/when spec stabilizes. That might be a while. I suggest we convert to draft / close until then.

[arnabnandy7]

I suggest we convert to draft / close until then.

@jack-berg I've converted it to a draft.