Configure kafka metrics reporter as class

[laurit]

Resolves #10823
Resolves #9883

[trask]

I didn't follow why test needs to use the internal MetricsReporterList?

[laurit]

Because we have a test that tries to serialize the configuration

opentelemetry-java-instrumentation/instrumentation/kafka/kafka-clients/kafka-clients-2.6/library/src/test/java/io/opentelemetry/instrumentation/kafkaclients/v2_6/internal/OpenTelemetryMetricsReporterTest.java

Line 132 in 17b6cad

void serializableConfig() throws IOException, ClassNotFoundException {

and that test gets the configuration from this method. If we just use an ArrayList here then the configuration will contain the list that includes OpenTelemetryMetricsReporter.class which will fail in

opentelemetry-java-instrumentation/instrumentation/kafka/kafka-clients/kafka-clients-2.6/library/src/test/java/io/opentelemetry/instrumentation/kafkaclients/v2_6/internal/OpenTelemetryMetricsReporterTest.java

Line 159 in 17b6cad

throw new IllegalStateException(

This is a bit convoluted, but the current behavior that depends on the thread context class loader being able to load the OpenTelemetryMetricsReporterTest isn't ideal either.