Histogram Aggregation - core SDK - aggregator

[beanliu]

under the suggestion of @bogdandrutu, this PR is splitted from #2581 and adds histogram aggregator into the core metrics SDK.

benchmark data of the aggregators from my laptop:

JMH version: 1.27
VM version: JDK 13.0.2, OpenJDK 64-Bit Server VM, 13.0.2+8
VM invoker: /Library/Java/JavaVirtualMachines/adoptopenjdk-13.jdk/Contents/Home/bin/java
VM options: -Xmx4G
JMH blackhole mode: full blackhole + dont-inline hint
Warmup: 5 iterations, 1 s each
Measurement: 10 iterations, 1 s each
Timeout: 10 min per iteration
Benchmark mode: Average time, time/op

Benchmark	Mode	Cnt	Score	Error	Units
DoubleHistogramBenchmark.aggregate_1Threads	avgt	10	29.178	± 3.746	ns/op
DoubleMinMaxSumCountBenchmark.aggregate_1Threads	avgt	10	30.218	± 1.824	ns/op
LongMinMaxSumCountBenchmark.aggregate_1Threads	avgt	10	26.643	± 0.571	ns/op
DoubleHistogramBenchmark.aggregate_5Threads	avgt	10	190.119	± 17.777	ns/op
DoubleMinMaxSumCountBenchmark.aggregate_5Threads	avgt	10	226.926	± 8.637	ns/op
LongMinMaxSumCountBenchmark.aggregate_5Threads	avgt	10	199.217	± 9.998	ns/op
DoubleHistogramBenchmark.aggregate_10Threads	avgt	10	342.492	± 6.906	ns/op
DoubleMinMaxSumCountBenchmark.aggregate_10Threads	avgt	10	457.424	± 37.492	ns/op
LongMinMaxSumCountBenchmark.aggregate_10Threads	avgt	10	489.890	± 1.476	ns/op

[jkwatson]

Thanks!