Fix Avg/Sum over Datetime column

[nablaone]

Our queries fail if we try to do sum or avg over a string or date column. This PR detects and compensates (adds a hack) it.

[trzysiek]

I think we should allow those functions on DateTime*, as they work in Elastic

We even have support for that in code, e.g. in model/metrics_aggregations/common.go - the response json is different for dates than normally. It used to work, we must have some regression.

But anyway I'd do that in another PR. I can do that if you want, should take only a few mins.

[trzysiek]

Looks good! But if I understand correctly from the code, we will allow such a request, but will return NULL (result of avg/sum(NULL))?
It's fine, so far we probably can't do better, but I'll also change that soon to return error to Kibana just after parsing query (after #1006 and some follow-up) so that we behave like Elastic