From review feedback:
* s/helper/metadata
* combined positional args to _request into options dict
* metadataRequest reuses _request()
* moved consumption of this.httpMethod into _request, can be overwritten
in options due to spread-after
* When set to `true`, the `silent` option for backend_srv requests
suppresses all event emitters that are triggered when the response is
received.
* Added `helperRequest()` to the Prometheus datasource to support
requests that are not triggered by the user, e.g., for tab completion.
`helperRequest()` sets the `silent` option.
* Migrated all non-timeseries queries of the Prometheus datasource to
use `helperRequest()`.
Fixes#11673
