Document Postgres Surfacer Table Structure

[theduke]

Unless I'm missing it, there is no documentation for how the table must look like for the Postgres surfacer.

[manugarg]

We should definitely document it. For a quick reference though, postges surfacer supports two schemas:

• You dump all the labels in JSON format in a single column.
• You pre-define a label to column mapping through surfacer's labelToColumn field and create schema according to that.

Here is what the schema will look like for the first option:

CREATE TABLE metrics (
    time timestamp,
    metric_name varchar(80),
    value float8,
    labels jsonb
);

[manugarg]

Added some documentation to the config itself:
https://cloudprober.org/docs/config/surfacer/#cloudprober_surfacer_postgres_SurfacerConf