vmui: Use POST instead of GET, to support long queries

[nick-kentik]

Is your feature request related to a problem? Please describe

We run a VictoriaMetrics cluster behind a haproxy load-balancer. I like using VMUI (also behind that load-balancer), but it fails to execute long queries due to restrictions the load balancer places on the request URI, including the query string.

VMUI sends a GET request with a request URI like .../select/0/prometheus/api/v1/query_range?query=..., which for some queries can get to 12KiB or so. At that size, haproxy returns a text/html 400 bad request and vmselect never sees the request.

Describe the solution you'd like

VictoriaMetrics supports both POST and GET to its query API for precisely this reason - the maximum length of a URI is not well defined, and it's easy to trip well-meaning limits imposed by intermediaries. I'd like VMUI to switch from making GET requests to POST requests so it works seamlessly in such an environment.

Grafana also offers the option of POST vs GET in its Prometheus datasource configuration - and defaults to POST, reserving GET for older versions of Prometheus that don't support it.

Describe alternatives you've considered

We could modify our load-balancer so it supports very long request URIs, but that would only help us, rather than everyone.

Additional information

Just a screenshot of a GET request made by VMUI: