Fix issue where annotations would not be returned for sharded queries

[charleskorn]

What this PR does

This PR fixes an issue where some annotations would not be returned for sharded queries.

This would occur if the annotation was generated during the evaluation of one of the shards.

For example, the query sum(rate(foo[5m])) should return a metric might not be a counter, name does not end in _total/_sum/_count/_bucket: "foo" info annotation. When this query is sharded, the annotation is generated during the evaluation of one of the sum(rate(foo{__query_shard__="x_of_y"}[5m])) shards on queriers, but the query-frontend would not combine annotations from individual shards into the final result.

This PR changes the behaviour of query-frontends to accumulate and return annotations from each sharded query.

Which issue(s) this PR fixes or relates to

(none)

Checklist

• Tests updated.
• [n/a] Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [n/a] about-versioning.md updated with experimental features.