Extend the resource-timing collection in the EUM server to be able to specify tags

[mariusoe]

Currently, the EUM server supports the collection of resource timings which can be turned on and off. The resource timings will be tagged using the global tags derived by the incoming Boomerang beacon. However, there is no option to specify additional tags next to the global ones.

This leads to the problem that in case there is no global tag containing the beacon's origin URL, the resource timings cannot be mapped to a specific resource.

In order to solve this problem, users should be able to specify additional tags which will be attached to the resource timings (similar to the metric view definition). For example, users should be able to specify a list of tags which will be attached. In the following example, the beacon attribute URL will be used as a value for the tag URL.

inspectit-eum-server:
  resource-timing:
    enabled: true
    tags:
      - URL