* add `correlationId` to queries
* trace correlation id in backend
* add correlation id to loki's span
* add correlation id to query chunks
* fix test
* fix DataSourceWithBackend test
* change to `queryGroupId`
* remove empty line
* fix test in `DataSourceWithBackend`
`X-Dashboard-Uid`, `X-Datasource-Uid`, `X-Grafana-Org-Id`, `X-Panel-Id` are very useful headers set
by Grafana front-end that we would like to see on the data source as
well. This is so that it would be possible to pinpoint from where slow
queries are coming in Mimir/Thanos/Cortex/etc., for example. Relevant
Mimir code lines:
0a94f26203/pkg/frontend/transport/handler.go (L182-L184)
Tested manually that with these changes the headers are visible.
