Feature: add option to export multiple pgbouncer instances

[maikelpoot]

Why this PR

We TCP load balance our connections over multiple (4+) pgbouncer instances/processes to increase stability and performance. We would like to switch from our own exporter to the prometheus-community exporter but would like to run only one exporter instance instead of an exporter per pgbouncer instance.

What does the PR do

This PR will add the option to export multiple pgbouncer instances with one exporter instance using a yaml config file.
All the options are documented within the example config.yaml within this repo.
This config file also add the option to start the exporter even if the connection fails on startup.

Extra labels

To differentiate the metrics for each pgbouncer instances, you MUST configure a unique label/value set for each instance.
As shown in the example, this can be as simple as setting a unique pgbouncer_instance value for each instance.

The extra labels per instance are merged with the common extra_labels where the instance value has precedence over the common value

Requirements:

• Every instance must define the same labels
• Every instance must define at least have one label with an unique value.

If there requirements are not met the exporter will fail startup.

Example

When using the config file:

must_connect_on_startup: false
extra_labels:
  environment: prod
pgbouncers:
  - dsn: postgresql://user:pass@localhost:6432/pgbouncer?sslmode=disable
    extra_labels:
      pgbouncer_instance: set1-0
  - dsn: postgresql://user:pass@localhost:6433/pgbouncer?sslmode=disable
    extra_labels:
      pgbouncer_instance: set1-1
  - dsn: postgresql://user:pass@localhost:6434/pgbouncer?sslmode=disable
    extra_labels:
      pgbouncer_instance: set1-2

you now get 3 pgbouncer_up metrics:

% curl -s http://localhost:9127/metrics | grep "_up"
# HELP pgbouncer_up The pgbouncer scrape succeeded
# TYPE pgbouncer_up gauge
pgbouncer_up{environment="prod",pgbouncer_instance="set1-0"} 1
pgbouncer_up{environment="prod",pgbouncer_instance="set1-1"} 1
pgbouncer_up{environment="prod",pgbouncer_instance="set1-2"} 0

[maikelpoot]

Force-push was to add sign-off to the commit

[maikelpoot]

@SuperQ Can i do something to add some progress the review process?

[garima-trivedi]

@SuperQ : This is definitely a useful and most wanted feature.
Or is there any other PR which is going to provide same feature. i.e. "Support to export metrics from multiple pgbouncer instances"
Please, can we expedite support for this feature?