* replacer: evaluate format once and improve perf by ~3x
This improves the performance of logging by almost 3x and reduces memory
usage by ~8x.
Benchmark results:
benchmark old ns/op new ns/op delta
BenchmarkReplacer-12 644 324 -49.69%
BenchmarkReplacer_CommonLogFormat-12 4228 1471 -65.21%
benchmark old allocs new allocs delta
BenchmarkReplacer-12 8 2 -75.00%
BenchmarkReplacer_CommonLogFormat-12 51 17 -66.67%
benchmark old bytes new bytes delta
BenchmarkReplacer-12 240 48 -80.00%
BenchmarkReplacer_CommonLogFormat-12 3723 446 -88.02%
* replacer: code review comments
* bufPool: document why we use a pointer to a slice
* parseFormat: fix confusing comment
* TestParseFormat_Nodes: rename to TestParseFormatNodes
* replacer: use a value for bufPool instead of a pointer
* replacer: remove comment
* replacer: replace labels with knownLabels
The previous slice of label names is no longer needed.