Series API should support match[] encoding

[owen-d]

Currently Prometheus uses match[] to encode multiple values, whereas Loki uses multiple match clauses in a querystring such as match=<>&match=<>. This stems from there being multiple ways to encode a list of values for the same key in a querystring. Our current approach works well with i.e. curl, but does not work with promtool.

Therefore, we should handle both encoding types.

Reference:

loki/pkg/loghttp/series.go

Line 20 in 7f61f11

xs := r.Form["match"]

https://github.com/prometheus/client_golang/blob/aa9238db679fc02bf22cf4f2c27a980edcb5ada0/api/prometheus/v1/api.go#L713

@yeya24 Thanks for the digging here.