* Vendor dependencies.
This updates several old dependencies, removes
some that are no longer needed, and adds
`pkg/labels` from prometheus `dev-2.0` branch.
* Add metrics selector parsing code
This is a temporary simplified re-implementation
of promQL's metric selector parsing.
* Add alerts filtering
Filter alerts through `?filter=` query string.
* Add silences filtering
Filter silences through `?filter=` query string.
* Move `parse` to `pkg/parse`
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