Use []float64 for histogram boundaries, not []metric.Number

[jmacd]

Reading through the histogram aggregator was confusing because the boundaries are specified as []metric.Number, but despite this were expected to be floating point numbers. This makes the boundaries []float, which is less confusing.

This also adds a benchmark. I considered whether binary search outperforms linear search in any practical-sized histogram, and the answer is no. Binary search beats linear search for histograms sized >= 256 to 512 elements.

Resolves #534.
Resolves #759.

[jmacd]

CURRENT:

BenchmarkHistogramSearchFloat64_1-8      	46579974	        25.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_8-8      	49497470	        31.8 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_16-8     	36765841	        32.8 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_32-8     	32826470	        34.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_64-8     	28035928	        40.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_128-8    	22074931	        54.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_256-8    	14861863	        82.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_512-8    	 8601362	       132 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_1024-8   	 5037547	       244 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_1-8        	53172440	        23.7 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_8-8        	45193213	        28.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_16-8       	39541150	        29.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_32-8       	36276222	        33.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_64-8       	29840234	        36.9 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_128-8      	22053164	        51.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_256-8      	15829436	        77.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_512-8      	 9319368	       129 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_1024-8     	 5245293	       242 ns/op	       0 B/op	       0 allocs/op

WITH BINARY SEARCH:

BenchmarkHistogramSearchFloat64_1-8      	39085161	        26.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_8-8      	29167771	        42.8 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_16-8     	22880655	        48.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_32-8     	20022715	        58.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_64-8     	16920318	        67.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_128-8    	15097021	        78.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_256-8    	13093621	        90.7 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_512-8    	11439160	       108 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchFloat64_1024-8   	 9849621	       121 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_1-8        	43069017	        29.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_8-8        	29564926	        45.2 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_16-8       	23641772	        61.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_32-8       	19429113	        65.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_64-8       	16292440	        72.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_128-8      	14794202	        81.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_256-8      	12253422	        94.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_512-8      	10108302	       118 ns/op	       0 B/op	       0 allocs/op
BenchmarkHistogramSearchInt64_1024-8     	 9784033	       125 ns/op	       0 B/op	       0 allocs/op

[MrAlias]

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