Explore: Query field fixes

Fix misspelled authentication in Auth overview doc

Don't use unnest in queries for redshift compatibility

Explore: Fix click to filter for recording rule expressions

- recording rule names contain ':'
- include this in the pattern for metric names

Fix megacheck issues

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/tsdb/elasticsearch/client/client.go:147:13:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)
pkg/tsdb/elasticsearch/client/client.go:190:14:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)
pkg/tsdb/elasticsearch/client/client.go:218:13:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/plugins/dashboards_updater.go:51:2:warning: 'if err != nil { return err }; return nil' can be simplified to 'return err' (S1013) (megacheck)

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/services/sqlstore/alert_notification.go:242:3:warning: 'if err != nil { return err }; return nil' can be simplified to 'return err' (S1013) (megacheck)

disable codecov

removes unused files

Fix setting test

ref #11433

moves files from /tests to more appropriate folders

Moves docker/ to devenv/docker

Explore: Add multiline syntax highlighting to query field

* brian2222-patch-1:
  removes testdata from getting started
  Update getting_started.md

ref #11433

Internal metrics: starts counters at zero

annotations: allows template variables to be used in tag filter

Add documentation for postgres query builder

Fix some megacheck issues

ref #11433