TSDB and scraping: improvements to dedupelabels

[bboreham]

Fixing various things that lead to memory growth when built with -tags dedupelabels, as reported at https://github.com/prometheus/prometheus/pull/12304#issuecomment-2024756897. Partner to #14322

• Scraping: add metric for symbol table size
• Scraping: better resetting of symbol-table when it grows
• Scraping: drop series cache when resizing symbol table
• TSDB: rebuild labels symbol-table on each compaction
• TSDB: reset symbol table for exemplars periodically

The larger additions are put into a new file which only builds with -tags dedupelabels, to avoid potential confusion and overhead when built in other ways.

[beorn7]

@jesusvazquez @codesome it was decided one of you should review this. :grin:

[bboreham]

Force-pushed to fix merge conflicts and squash intermediate commits.