Optimize meter Id tag storage

[fvallenilla]

Summary

• store Id tags as ordered pairs instead of copying maps through each clone path
• keep the line-protocol and builder changes from the base PR and make WithTag/WithTags cheaper on top
• add focused Id tests for clone, upsert, sorted, and toMap behavior

Benchmark results

Baseline: #124

name                           old time/op    new time/op    delta
CounterAddCombined-12            492.8ns       398.2ns      -19.21%
CounterAddCombinedPostGC-12      586.8ns       487.9ns      -16.84%
geomean                          537.7ns       440.8ns      -18.03%

name                           old B/op       new B/op      delta
CounterAddCombined-12             768.0         608.0       -20.83%
CounterAddCombinedPostGC-12       769.0         609.0       -20.81%
geomean                           768.5         608.5       -20.82%

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/meter -run '^$' -bench 'BenchmarkCounterAddCombined$|BenchmarkCounterAddCombinedPostGC$' -benchmem -count=10

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