Resolve 'rt.bmr' data send by Boomerang plugin RT

[Patrick-Eichhorn]

The data contained in the 'rt.bmr' attribute, is a comma separated list.

See: http://akamai.github.io/boomerang/BOOMR.plugins.RT.html#toc1__anchor

rt.bmr: Several parameters that include resource timing information for boomerang itself, ie, how long did boomerang take to load

Example: rt.bmr="37,1,2,2"

Each of these values should be available during the metric and tag processing.
As an example, the previous data could result in attributes similar to the following, divided according to their index:

rt.bmr.0: 37
rt.bmr.1: 1
rt.bmr.2: 2
rt.bmr.3: 2
rt.bmr.sum: 42