Add server tag to metrics and service_check

[ian28223]

What does this PR do?

adds pg_instance:<host|host-port> tag to metrics to distinguish which instance the metric came from.

Given a host (e.g. dd-host1) that connects to multiple remote PG instances:

instances:
  - host: /var/run/postgresql/.s.PGSQL.5432
  - host: pg01
    port: 5432
  - host: pg02
    port: 5433

metric submitted from each instance are identical and has no reference to which pg instance they were queried from.

{
  "metric": "postgresql.table.count",  "points": [[ 1547173475, 1]],
  "tags": [
    "schema:public"
  ],
  "host": "dd-host1",
  "type": "gauge", "interval": 0, "source_type_name": "System"
}

With the pg_instance tag, it would look like:

[
  {
    "metric": "postgresql.table.count", "points": [[ 1547173475, 1]],
    "tags": [
      "pg_instance:/var/run/postgresql/.s.PGSQL.5432",
      "schema:public"
    ],
    "host": "dd-host1",
    "type": "gauge", "interval": 0, "source_type_name": "System"
  },
  {
    "metric": "postgresql.table.count", "points": [[ 1547173475, 1]],
    "tags": [
      "pg_instance:pg01-5432",
      "schema:public"
    ],
    "host": "dd-host1",
    "type": "gauge", "interval": 0, "source_type_name": "System"
  },
  {
    "metric": "postgresql.table.count", "points": [[ 1547173475, 1]],
    "tags": [
      "pg_instance:pg02-5433",
      "schema:public"
    ],
    "host": "dd-host1",
    "type": "gauge", "interval": 0, "source_type_name": "System"
  }
]

Motivation

Realized that i could not distinguish which metrics are coming from what instance.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If PR adds a configuration option, it has been added to the configuration file.

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2928 into master will increase coverage by 0.4%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2928      +/-   ##
==========================================
+ Coverage   85.44%   85.85%    +0.4%     
==========================================
  Files         676        9     -667     
  Lines       35517      735   -34782     
  Branches     4205      104    -4101     
==========================================
- Hits        30348      631   -29717     
+ Misses       4016       73    -3943     
+ Partials     1153       31    -1122

[gzussa]

@ChristineTChen @masci Any feedback regarding this PR?

[ian28223]

rabased and made some adjustments due to some conflicts

[gzussa]

Nice one!

[zippolyte]

Followed up by #3467