Support for manual timings and merging/adding timings

[luzat]

I feel that the interface currently is a bit limited:

1. How would one measure time for database queries from multiple code locations? It should be possible to somehow merge multiple measurements for one timing, e.g. by adding up all dur values by name. This would not necessarily change the visible interface. Merging descriptions does not make much sense, so one could either use the first or last description that was passed in for a name or change the API such that it is possible to only pass description once for a given name.

2. Sometimes it would be easier to specify timings manually, e.g. when timings come from requests to other APIs that return timing information themselves or if code does its own measurements.

Internally, I would probably opt to differentiate active collectors and the current results grouped by name (Map).