added extraction of labels from JSON column

[samsk]

Hi, you might not like it, but I've implemented it works ;-)

This change introduces new metric option json_labels that specified a column that should be used to extract additional labels to metric, without the need to name them one by one in the query.
If some style/whatever changes needed to merge it, I'll make them.

[free]

Thanks for writing the code. I'm not going to accept the PR, because I believe there are easier ways around it, plus a number of other reasons.

First, as you put it, JSON is a first class citizen in most databases nowadays, so you should be able to extract the label values in your query, just as you would select values from a regular table column.

Second, if the argument is that you can't have fully dynamic labels for your metrics, that is by design. Your data should not control what is essentially your schema. E.g. you don't want to have a metric with 3 labels and another metric, with the same name, but 4 labels. (I seem to recall that the Prometheus client library would blow up if you try to do that.) Labels should be configuration controlled, not data controlled.

That being said, I did add some comments to clean up the code. And I'll leave the PR around, in case someone else is interested.

[samsk]

1. yes you can, but than you have to change configuration. I kind a like it, when everything about database is declared in database, even what is monitored and what labels it has - ie. if there is bool metric (ok/down), than the evaluation logic should be in database view and not in the query in some config file - and if it is so, why not to extend this logic to labels ;-)

2. Regarding fully dynamic labels - I know that prometheus will refuse different labels for the same metric and that is correct and expected. They should not be absolutely dynamic.

But ok, thanks for your review.