Implement multitarget pattern support

[maikelpoot]

note: this is a follow-up to #182

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 the multitarget pattern defined by prometheus.
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 when used in legacy mode (single-target).

Example

The probe endpoints accepts 2 parameters:

• dsn: the postgresql connection string
• cred: credential reference to credentials stored in the config file.

When cred is used the dsn will be updated with the credential values from the config file.
If the DSN and credentials config define the same parameter the latter takes precedence.
With the example below the exporter will scrape "postgres://username:password@localhost:6543/pgbouncer?sslmode=disable".

prometheus.yaml:

- job_name: pgbouncer-exporter
  metrics_path: /probe
  params:
    dsn: "postgres://localhost:6543/pgbouncer?sslmode=disable"
    cred: "monitoring"

config.yaml

credentials:
  - key: monitoring 
    username: username
    password: password

[SuperQ]

The metric_path and probe_path need to be removed from the configuration file.

Since these are not reloadable configurations, we want them to be command line arguments only.

[SuperQ]

There should be no "legacy mode". Both normal scrape and probe endpoints should be supported simultaneously.

[maikelpoot]

If i read (and interpret) the multi-target docs correctly, a multi-target exporter should only export it's own metrics over /metrics and /probe should be used for target metrics.

How do you see using both mode's simultaneously? Should /metrics contain the metrics for the configured target or should /probe without parameters return these?