Add coverage of keys() to common test frameworks

[jaydeluca]

any idea how we could get coverage of keys() via the common http server tests, so it would cover all of the instrumentations?

As discussed here

from @laurit :

We could introduce a custom verifying propagator that calls the keys() method. We could wire it up in https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/8481d9a550652c97d2a16864064730dc5a487612/testing-common/src/main/java/io/opentelemetry/instrumentation/testing/LibraryTestRunner.java#L105-107 and in https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/testing/agent-exporter/src/main/java/io/opentelemetry/javaagent/testing/exporter/AgentTestingCustomizer.java

[trask]

Goal would be to have common coverage across all TextMapGetters and be able to delete WebfluxTextMapGetterTest.java.