* Metrics: remove unused metrics
Metric `M_Grafana_Version` is not used anywhere, nor the mentioned
`M_Grafana_Build_Version`. Seems to be an artefact?
* Metrics: make the naming consistent
* Metrics: add comments to exported vars
* Metrics: use proper naming
Fixes#18110
* fix label handling
* SEARCH() results could have multiple namespace, remove from legend
* divide GetMetricStatistics related code
* divide GetMetricData related code
* divide parseGetMetricDataResponse()
* divide parseGetMetricDataQuery()
* divide test code
* add test for GetMetricData
* add test for GetMetricData parse response
* fix bug of terminating gap
* fix gofmt
Oleg Gaidarenko
Mitsuhiro Tanda