Implement metadata API limit param

[harry671003]

Is your proposal related to a problem?

The metadata APIs like /api/v1/series, /api/v1/labels, /api/v1/label/<name>/values often timeout when queried with a lot of data.
Recently Prometheus introduced a new limit param in the above APIs for limiting the amount of data returned. This change was implemented in:

• web/api add limit param prometheus/prometheus#13396
• storage: pass limit param as hint in querier prometheus/prometheus#14109

However, we currently don't make use of this information, so the stores may do a bunch of work retrieving values that will then be thrown away in the querier.

Describe the solution you'd like

Thanos stores should truncate the results if the limit param is set and return only the required number of results

Describe alternatives you've considered

(Write your answer here.)

Additional context

prometheus/prometheus#12795 has more context about this problem.

Work tracker

• Implement limit param in APIs
• Implement limit param in stores
• Smaller improvements
• Acceptance tests