Per-service metrics resolution improvements

[sleto-it]

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Please do not merge until the recent readme.io updates are reverted.

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Issues:

• The version from where the feature was available was not mentioned. Now fixed.
• The link to the API was wrong. Now fixed.
• In general, the mentioned API was just an example, for Postgres. It was important to document things in a more general way, so that things can be applied for MySQL and Mongo as well. Done
• The doc was too generic. The specific metrics_resolutions parameter is now mentioned, the procedure has been described, and an API call example has been added.
• How to reset the customization was not documented. Done now