max_returned_metrics: parse string value with fallback

[tbavelier]

What does this PR do ?

This PR allows the use of integer string value for the max_returned_metrics parameter. If the string cannot be parsed, falls back to default limit instead.

Motivation

Recent customer case where their JMX configuration would not work once they added max_returned_metrics. As they were using k8s annotations, they tried to use a string in the JSON instead of the integer, causing the instance not to be loaded with the error message Unable to create instance. Please check your yaml file :

{
  "instances": [
    {
      "host": "%%host%%",
      "port": 9999,
      "max_returned_metrics": "2000"
    }
  ]
}

instead of

{
  "instances": [
    {
      "host": "%%host%%",
      "port": 9999,
      "max_returned_metrics": 2000
    }
  ]
}

With this PR, both syntaxes would work.