Search: drop use of TagCache, extract tags and tag values on-demand

[yvrhdn]

What this PR does:
Instead of storing tags and tag values in the TagCache we can extract them directly from live traces, WAL and local blocks when requested. This should lower the amount of work spent on updating the tagcache during ingest.

Since tag values are extracted on-demand, we are not limited anymore by the size of the tag cache. So this makes it possible to return thousands of values when needed.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yvrhdn]

I've been doing a test on my local set up fetching tag values for load_generator.seq_num, this is a unique ID added to every trace by the synthetic-load-generator. Before these changes the amount of values always got truncated at about ~200 values. With this PR amount of values returned increases up to 80k and drops back to 20k when a block was flushed.

Avg latency sits around 200-400 ms.

[yvrhdn]

TODO

• Check if the context has been cancelled
  
• Experiment with filtering tag values on service.name Next PR
• Most time is spent searching live traces, can we speed this up?

[yvrhdn]

Still to address: when fetching tag values with very high cardinality (for instance http.url) requests fail/time out when being sent between components. I.e. the ingester is fine digging up all the values, but the GRPC connection between the ingester, querier and query-frontend was hitting the max bytes size...
We should add some limits (optionally configurable) to protect against this happening. Ideally, we return early when we have 'a lot of' values.

Future improvements (not for this PR): we can add filter capabilities to the tag values endpoint, so if someone starts typing http.url=/myService/ we only return values containing /myService/. This would make high cardinality tags more useable.

[joe-elliott]

Remove the TagCache struct?

[yvrhdn]

Remove the TagCache struct?

Yep 👍, I've removed tempodb/search/tag_cache.go, it's all the way at the bottom of the diff
https://github.com/grafana/tempo/pull/1068/files#diff-b29b479cf004a8bf08fb55114702b94f6d818911fca0c83c985e771db2fbe4bb

[mdisibio]

Thanks for getting the limit check in. Good to merge.