[Metrics] Reduce CPU consumption of metrics creation

[lhotari]

Motivation

String.format is consuming most CPU when generating metrics:

(Flamegraph from Pulsar user, using Pulsar 2.7.0)

Besides direct CPU consumption, it creates a lot of garbage which then causes more CPU being spent in GC.

Modifications

It is possible to get rid of the key creation and avoid String.format calls and String object allocation, by using a similar approach as @rdhabalia used in NamespaceStats:

pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/stats/NamespaceStats.java

Lines 46 to 63 in cc64889

	static {
	// create static ref for add-latency-bucket keys to avoid new object allocation on every stats call.
	for (int i = 0; i < ENTRY_LATENCY_BUCKETS_USEC.length + 1; i++) {
	String key;
	// example of key : "<metric_key>_0.0_0.5"
	if (i == 0 && ENTRY_LATENCY_BUCKETS_USEC.length > 0) {
	key = String.format("%s_0.0_%1.1f",
	BRK_ADD_ENTRY_LATENCY_PREFIX, ENTRY_LATENCY_BUCKETS_USEC[i] / 1000.0);
	} else if (i < ENTRY_LATENCY_BUCKETS_USEC.length) {
	key = String.format("%s_%1.1f_%1.1f",
	BRK_ADD_ENTRY_LATENCY_PREFIX, ENTRY_LATENCY_BUCKETS_USEC[i - 1] / 1000.0,
	ENTRY_LATENCY_BUCKETS_USEC[i] / 1000.0);
	} else {
	key = String.format("%s_OVERFLOW", BRK_ADD_ENTRY_LATENCY_PREFIX);
	}
	ADD_LATENCY_BUCKET_KEYS[i] = key;
	}
	}

.
Since there are multiple sets of buckets, there is a need for a class that handles the logic.

[eolivelli]

awesome

[michaeljmarshall]

Good find. LGTM.

[lhotari]

/pulsarbot run-failure-checks

[lhotari]

/pulsarbot run-failure-checks