fix: Reduce allocations for classic histogram buckets

[jaydeluca]

Related to #2075

This reduces allocation and GC pressure in histogram text formatting by improving buffer reuse and eliminating unnecessary intermediate allocations.

I added and ran some benchmarks:

Histogram text formatting

Benchmark	Before (ops/s)	After (ops/s)	Δ Throughput	Before (B/op)	After (B/op)	Δ Allocation
openMetricsReusingWriter	14,487	17,882	+23%	76,128	18,192	-76%
openMetricsWriteToByteArray	14,338	16,588	+16%	77,088	43,648	-43%
prometheusReusingWriter	14,748	18,007	+22%	69,256	18,264	-74%
prometheusWriteToByteArray	14,333	16,588	+16%	76,288	43,720	-43%

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GC impact

Benchmark	Before GC Count	After GC Count	Before GC Time	After GC Time
openMetricsReusingWriter	54	7	101 ms	16 ms
prometheusReusingWriter	50	7	81 ms	17 ms

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