feat: add {emitToString, emitToBuffer} options + dedup TYPE and HELP per metric name

Add {emitToString, emitToBuffer} thread-safe internal buffer management variants to emit metrics.

Deduplicate TYPE and HELP lines per metric name by default on emit + enforce same HELP per metric name.

Signed-off-by: Melissa Kilby <[email protected]>

Author: incertum