Fill Labels attribute of a DataFrame Field.

[stephan-hof]

Hi, my goal is support for alerting on multiple series.

This issue has been reported in #362. See point two with the reference to grafana/grafana#7832

It appears that grafana uses the Labels of a Field within a DateFrame to distinguish/track the different alerts. Currently the clickhouse-grafana plugin leaves the Labels empty.
So grafana does not know how the multiple DataFrames differ and shows the following error instead:

invalid format of evaluation results for the alert definition: 
frame cannot uniquely be identified by its labels: has duplicate results with labels {}

In this pull request I fill the Labels attribute and now alerting of multiple series works as expected.
See the following screenshot. The important bit as at the end after the 'Preview Alerts'.
There is a list with the different series, based on the labels. Which means the grafana alert tracking looks at the different series independently.

Without this fix the alert would raise this error instead.

[Slach]

thanks a lot for contributing