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.

[cfredri4]

From the spec discussion:

open-telemetry/opentelemetry-specification#5150 (comment)

You can also consider making Keys an optional functionality (in OTel Java) that the Getter does not have to implement (or return some default implementation).

It is worth mentioning that the spec DOES NOT require implementing Keys.

One of the ways to implement it is Getter class with methods Get, Keys, and GetAll as described below. Languages may decide on alternative implementations and expose corresponding methods as delegates or other ways.

So until the spec discussion is settled, can the TextMapGetter.keys() get a default empty implementation, but not be deprecated?

[jkwatson]

I have no opinion one way or another on this one. I understand the appeal of being able to make this a functional interface, but it seems not good to have an empty iterable be the default, as that could end up being confusing for consumers. I'm fine with this either way.