Async Metrics

[felixdesouza]

Goals (and why):
As part of some of the conjure-undertow changes, async versions of requests were added. The sad part is, the metrics as we know currently are off since they measure the time it takes to be given back the ListenableFuture vs how long the ListenableFuture takes to complete.

Ideally this would go in https://github.com/palantir/tritium, but given that we've hacked together the TaggedMetricsInvocationHandler for our own (nefarious?) gains, I didn't want to unwind that change as I'm working on Timelock Partitioning etc. @schlosna for SA. My proposed impl would be that it goes in ByteBuddyInstrumentationAdvice or InvocationEventProxy then every InvocationHandler gets async metrics stuff for free!

Implementation Description (bullets):

• Backport some of the metrics changes in the TaggedMetricsInvocationHandler wrt metric name cache etc. Since the metric name doesn't change for most invocations we're wasting time doing the string concat and the allocations for MetricName. For the ones with the extra tags, have a specialised cache for those calls.
• Check if the return type is ListenableFuture and if so, add a listener that will update the metric. Otherwise we leave it as is.

Testing (What was existing testing like? What have you done to improve it?):

• Hard to test properly since we can't change the future that gets returned, we can only act on it => we can have a race condition where if we were to wait for the future to be done, it's not necessarily the case that the metrics would have been tracked correctly. Only fix I see for this is in upstream in tritium.

Concerns (what feedback would you like?):

Where should we start reviewing?:

• TaggedMetricsEventInvocationHandler
• AtlasDbMetrics

Priority (whenever / two weeks / yesterday):
ASAP! Some metrics in our internal dashboard is a lie until fixed.

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Methods returning ListenableFutures now have their metrics accurately tracked.

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[Jolyon-S]

I think the semantics are good, just a few stylistic comments.

[Jolyon-S]

LGTM.