Measure response time and provide access through metrics

[3Hren]

This is not an easy task, because I haven't found exponentially decaying histograms for Rust.

[karitra]

I could try to implement one modelled after same from libmetrics in spare time if you feel brave enough, it would be a nice Rust exercise for me.

[3Hren]

Ok!

[3Hren]

Actually it seems like it is pretty heavyweight and broad metric. Need to find out how (and should ever?) can it be implemented.