Expose some basic catalog and config metrics to be exported by micrometer

[groldan]

Expose some basic catalog and config metrics to be exported by micrometer.

If spring-boot-actuator is present on the (GeoServer) application,
and the config propery geoserver.metrics.enabled=true (default),
the following metrics are exported through micrometer, with the
instance-id tag matching the info.instance-id value to ease
slicing by service when aggregating values on an external system.

• geoserver.config.update_sequence: The configuration update sequence.
• geoserver.catalog.added: Number of CatalogInfo objects added to
  this instance's Catalog
• geoserver.catalog.removed: Number of CatalogInfo objects removed
  on this instance's Catalog
• geoserver.catalog.modified: Number of modifications to CatalogInfo
  objects on this instance's Catalog

They can then be monitored at each instance's actuator endpoint. For
example:

curl -i http://192.168.0.21:8106/actuator/metrics/geoserver.config.update_sequence
HTTP/1.1 200
Content-Type: application/vnd.spring-boot.actuator.v3+json

{
  "name": "geoserver.config.update_sequence",
  "description": "GeoServer configuration update sequence",
  "baseUnit": "sequence",
  "measurements": [
    {
      "statistic": "VALUE",
      "value": 208.0
    }
  ],
  "availableTags": [
    {
      "tag": "instance-id",
      "values": [
        "web-ui:192.168.0.21:9106"
      ]
    }
  ]
}

References:

• Spring Boot Metrics
• Micrometer