Thread byte writes through buffered writers

[fvallenilla]

Summary

• let line-protocol helpers keep their byte slices through the writer boundary via WriteBytes
• update buffered UDP and unixgram writer paths so byte writes preserve the same buffering semantics as string writes
• add a targeted write-path benchmark to compare Write(string) with WriteBytes([]byte) behavior

Benchmark results

Baseline: #124

name                           old time/op    new time/op    delta
CounterAddCombined-12            510.1ns       478.4ns       -6.21%
CounterAddCombinedPostGC-12      585.6ns       595.5ns          ~
geomean                          546.5ns       533.7ns       -2.35%

name                           old B/op       new B/op      delta
CounterAddCombined-12             768.0         592.0       -22.92%
CounterAddCombinedPostGC-12       769.0         593.0       -22.89%
geomean                           768.5         592.5       -22.90%

name                           old allocs/op  new allocs/op delta
CounterAddCombined-12              5.000         4.000      -20.00%
CounterAddCombinedPostGC-12        5.000         4.000      -20.00%
geomean                             5.000         4.000      -20.00%

Test plan

• go test ./spectator/meter
• go test ./spectator/writer -run '^$'
• go test ./spectator/meter -run '^$' -bench 'BenchmarkCounterAddCombined$|BenchmarkCounterAddCombinedPostGC$' -benchmem -count=10
• go test ./spectator/meter -run '^$' -bench 'BenchmarkWriteLineInt(Current|Bytes)' -benchmem -count=1

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