* Prometheus: Add metadata for summary metrics

- summary metrics don't have metadata available from the metadata API,
so Grafana can help and just add it
- given a summary metric `foo`, we add metadata info for `foo_sum` and
`foo_count`
- with tests

* Update public/app/plugins/datasource/prometheus/language_utils.ts

Co-authored-by: gotjosh <josue@grafana.com>

Co-authored-by: gotjosh <josue@grafana.com>