[prometheus-mysql-exporter] Add support for multiple target

[Remy-Mollandin-SK5]

Is your feature request related to a problem ?

mysqld_exporter support multiple target since the release 0.15.0. https://github.com/prometheus/mysqld_exporter/releases/tag/v0.15.0

The implementation use a new route named /probe and credentials for each target are managed in the config file config.cnf. The current chart doesn't allow serviceMonitor path overide or adding params.

Describe the solution you'd like.

An implementation close to prometheus-redis-export would be ideal: https://github.com/prometheus-community/helm-charts/blob/main/charts/prometheus-redis-exporter/values.yaml#L108

serviceMonitor:
  # When set true then use a ServiceMonitor to configure scraping
  enabled: false
  multipleTarget: false
  targets: []
  # for every targets, url and name must be set, port is optionnal
  # an individual additionalRelabeling can be set for every target
  # - endpoint: mysql1.dns.local
  #   name: mysql1
  #   port: 3307
  # - endpoint: mysql2.dns.local
  #   name: mysql2

The name could be used as key to retrieve the database credentials in config.cnf

[client.mysql1]
user=mysql1_user
password=mysql1_password
[client.mysql2]
user=mysql2_user
password=mysql2_password

Describe alternatives you've considered.

NONE

Additional context.

No response

[Remy-Mollandin-SK5]

#3751 drafted for implementation.

[Remy-Mollandin-SK5]

#3751 should be ready for review.

It's working fine on a project with 5+ databases per environment.