Added query cache metrics

[rafael]

Description

Exposed metrics for query cache.

Should we call this metric PlanCache* to make it consistent with the code?

I think it makes more sense to keep it like this (QueryCache*), so it's consistent with the one in tabletserver. But curious to read what people think about this.

Tests

• Locally and dev cluster

[demmer]

I'd say PlanCache makes more sense since vtgate (nor vttablet for that matter) isn't actually caching queries, it's caching query plans.

[derekperkins]

Agreed. I did a double take when I first saw references to QueryCache, thinking that Vitess was supporting the deprecated MySQL QueryCache.

[bbeaudreault]

+1 for PlanCache. Would be great to change any docs that refer to it as QueryCache as well. We've had some newcomers recently who have been confused by this.

[rafael]

Sounds good. Updated. I'll open a different PR to update tablet server and docs related to that one.

[sougou]

LGTM