Improvement: Ignore beginning comma in query selectors

[JensRantil]

Background: When I (in the Prometheus web GUI) query metrics, I generally first query for a list of all metrics and then copy-paste some selectors to refine my query.

Problem: Copy-pasting many times mistakenly includes a beginning comma (,). my_metric{instance="my-instance",} is gracefully handled by Prometheus but my_metric{,instance="my-instance"} is not and yields Error executing query: parse error at char 11: unexpected "," in label matching, expected identifier.

Proposed solution: That my_metric{,instance="my-instance"} is interpreted as my_metric{instance="my-instance"} and Prometheus is a bit more forgiving :-)