Reduce meter line formatting allocations

[fvallenilla]

Summary

• route meter writes through shared line-protocol helpers instead of per-call fmt.Sprintf
• switch spectatord ID construction to a pooled byte builder while keeping Id map-backed in this first step
• add hot-path benchmarks for NewId -> NewCounter -> Add

Benchmark results

Baseline: main

name                           old time/op    new time/op    delta
CounterAddCombined-12           1020.0ns       500.1ns      -50.97%
CounterAddCombinedPostGC-12     1107.5ns       585.8ns      -47.11%
ToSpectatorIdBuilder-12          731.6ns       338.9ns      -53.69%
geomean                          938.5ns       463.0ns      -50.66%

name                           old B/op       new B/op      delta
CounterAddCombined-12            1128.0         768.0       -31.91%
CounterAddCombinedPostGC-12      1130.0         769.0       -31.95%
ToSpectatorIdBuilder-12           504.0         176.0       -65.08%
geomean                           862.9         470.2       -45.51%

name                           old allocs/op  new allocs/op delta
CounterAddCombined-12             12.000         5.000      -58.33%
CounterAddCombinedPostGC-12       12.000         5.000      -58.33%
ToSpectatorIdBuilder-12            6.000         1.000      -83.33%
geomean                            9.524         2.924      -69.30%

Test plan

• go test ./spectator/meter
• go test ./spectator/meter -run '^$' -bench 'BenchmarkCounterAddCombined$|BenchmarkCounterAddCombinedPostGC$|BenchmarkToSpectatorIdBuilder$' -benchmem -count=10

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