support multi-value model in Prometheus compatible way

[yuanbohan]

Purpose

Prometheus is a single-value model, it is to draw a single panel from query_range response by default. Since GreptimeDB supports multi-value model, but the compatible /api/v1/query_range endpoints can not facilitate multi-value scenarios.

HTTP response format

• value under values has multiple value, but the same format as Prometheus when one field is specified in PromQL, or only one column in the table
• fields tells the name of each field in the response

Instant Query response

{
   "metric" : {
      "__name__" : "up",
      "job" : "prometheus",
      "instance" : "localhost:9090"
   },
   "fields": ["field1", "field2", "field3"],
   "values" : [
      [ 1435781430.781, "1", "2", "3" ]
   ]
}

Range Query response

{
   "metric" : {
      "__name__" : "up",
      "job" : "prometheus",
      "instance" : "localhost:9090"
   },
   "fields": ["field1", "field2", "field3"],
   "values" : [
      [ 1435781430.781, "1", "2", "3" ],
      [ 1435781445.781, "1", "2", "3" ],
      [ 1435781460.781, "1", "2", "3" ]
   ]
}

Panel

The recommended way to draw the panels from multi-value response needs to be carefully designed. The potential options maybe:

• overlapped panels (the order of the panels can be changed)
• normal multiple panels (similar as repeat option in Grafana)

[waynexia]

cc @ZonaHex @alili for those who have experience in rendering those data.

My one concern is some fields might be missing or changed due to computation. Like max(metrics) may change the field name to MAX(metrics). So the actual field name will be different from what it was defined and is a runtime variable.