Use IdentityHashMap for consistent JIT optimization in AbstractCompositeMeter

[belugabehr]

Replace Collections.emptyMap() with new IdentityHashMap to maintain consistent map implementation type throughout the object's lifecycle, enabling JVM scalar replacement optimization and reducing iterator allocation overhead.

Fixes gh-6811

[belugabehr]

@jonatan-ivanov - Made suggested change. Confirmed the allocations have stopped: #6811 (comment)

[shakuzen]

@belugabehr could you share the code for the benchmark you're using?

[belugabehr]

@shakuzen Details are posted in: gh-6811

Thanks!

[shakuzen]

I think this change is fine as is, but it's also brittle. I wonder if we should try adding a test that asserts there is no allocation after C2 compilation. The problem is that we only have heuristics to guess when C2 compilation is done - by default in most modern JVMs it starts after 10,000 invocations of a method.

It may be helpful to commit the benchmarks along with this, but benchmarks don't get run with the build so they are not a guarantee this won't regress with some other change.

I think there is larger refactoring to consider around this, but I think it's okay to consider that after merging this.

I also think we should pursue adding a warning log so that users who can do all setup before registering metrics can avoid surprises like this and others (such as the values being recorded before a non-composite registry is added being dropped).

[shakuzen]

If we want to ensure a specific type for optimizations, it's probably best to use the specific type on the left-hand side here.

[belugabehr]

Static values like this cannot be templated and require this bridge. Please consider this PR as-is. If you would like a different variation, would you mind creating a new PR with whatever you think is best in order to streamline the back-and-forth? Thanks!

[shakuzen]

Suggested change

	private Map<MeterRegistry, T> children = (Map<MeterRegistry, T>) EMPTY_CHILDREN;
	private IdentityHashMap<MeterRegistry, T> children = (IdentityHashMap<MeterRegistry, T>) EMPTY_CHILDREN;

My comment wasn't clear enough. I should have used the code suggestion feature, but other changes are required along with it. Anyway, we can take care of it as polish when merging. I thought it was worth bringing up in review, though.

[belugabehr]

I also think we should pursue adding a warning log so that users who can do all setup before registering metrics can avoid surprises like this and others (such as the values being recorded before a non-composite registry is added being dropped).

Agreed!

[shakuzen]

I also think we should pursue adding a warning log so that users who can do all setup before registering metrics can avoid surprises like this and others (such as the values being recorded before a non-composite registry is added being dropped).

I've opened #6908 for this.

[belugabehr]

@shakuzen - PR updated with your suggestions. Thanks for all the help! Excited to get this over the line.