Add label field support for sub-queries

[skyairmj]

Hi,

Prometheus-Sql is very helpful, especially the sub-metrics approach. However, sub-metrics doesn't support label fields yet. It makes prometheus-sql not be able to handle multi-row multi-metric scenarios.

Giving a query string as:

select group, subgroup, count, avg from test_table

and the result set as bellow:

group	subgroup	count	avg
a	a1	10	5
b	b1	7	6

We'd like to register metrics as bellow:

metric	value
xxx_count(group = 'a', subgroup='a1')	10
xxx_avg(group = 'a', subgroup='a1')	5
xxx_count(group = 'b', subgroup='b1')	7
xxx_avg(group = 'b', subgroup='b1')	6

To achieve this, shall we treat all fields not specified under sub-metrics as label fields for all sub metrics? e.g.

- xxx:
    sql: >
        select group, subgroup, count, avg from test_table
    sub-metrics: # group, subgroup become label fields for both xxx_count and xxx_avg metrics as above
      count: count
      avg: avg
    interval: 30s

What's your opinion?

[Teriand]

But this work fine now.
ex.

- pg_statio_user_tables:
    data-source: pgmain
    sql: >
            SELECT 'MSK-MAIN' as stand, 'pgmain' as pg_instance, schemaname, relname,
            heap_blks_read, heap_blks_hit,coalesce(idx_blks_read,-1) as idx_blks_read, coalesce(idx_blks_hit,-1) as idx_blks_hit,
            coalesce(toast_blks_read,-1) as toast_blks_read, coalesce(toast_blks_hit,-1) as toast_blks_hit, coalesce(tidx_blks_read,-1) as tidx_blks_read, coalesce(tidx_blks_hit,-1) as tidx_blks_hit
            FROM pg_statio_user_tables
    interval: 5m
    sub-metrics:
      heap_blks_read: heap_blks_read
      heap_blks_hit: heap_blks_hit 
      idx_blks_read: idx_blks_read
      idx_blks_hit: idx_blks_hit
      toast_blks_read: toast_blks_read
      toast_blks_hit: toast_blks_hit
      tidx_blks_read: tidx_blks_read
      tidx_blks_hit: tidx_blks_hit

in metrics:

# HELP query_result_pg_statio_user_tables_heap_blks_hit Result of an SQL query
# TYPE query_result_pg_statio_user_tables_heap_blks_hit gauge
query_result_pg_statio_user_tables_heap_blks_hit{pg_instance="pgmain",relname="activator_message",schemaname="queue",stand="msk-main"} 4.0721516415e+10
query_result_pg_statio_user_tables_heap_blks_hit{pg_instance="pgmain",relname="alarms",schemaname="state",stand="msk-main"} 198160
query_result_pg_statio_user_tables_heap_blks_hit{pg_instance="pgmain",relname="alarms_sources",schemaname="state",stand="msk-main"} 203279

Only 1 features need prom-sql its custom HELP for replace "Result of an SQL query"

[haxorof]

Regarding help text, it is handled in #48