Provider better APIs to move between Instrument Tags and Commons Tags

[gregturn]

When defining a custom Observation, you get exposed to Commons TagKey...

enum MongoObservation implements DocumentedObservation {
}

This makes it tricky to use them in meter-based assertions like this:

@Test
void successfullyCompletedCommandWithoutClusterInformationShouldCreateTimerWhenParentSampleInRequestContext() {

	// given
	Observation parent = Observation.start("name", observationRegistry);
	TestRequestContext testRequestContext = TestRequestContext.withObservation(parent);

	// when
	listener.commandStarted(new CommandStartedEvent(testRequestContext, 0, null, "database", "insert",
			new BsonDocument("collection", new BsonString("user"))));
	listener.commandSucceeded(new CommandSucceededEvent(testRequestContext, 0, null, "insert", null, 0));

	// then
	assertThat(meterRegistry).hasTimerWithNameAndTags(HighCardinalityCommandTags.MONGODB_COMMAND.getKey(),
			Tags.of(Tag.of(LowCardinalityCommandTags.MONGODB_COLLECTION.getKey(), "user")));
}

Instead, you are forced to "convert" from one to the other.

If Commons Tags are, well, common, why can't Micrometer Tracing use them? Isn't that the point of a "common"?