Counter and gauge metrics

Added a counter and gauge functionality to the timings extras library. 

Similar to https://github.com/serilog/serilog/issues/20#issuecomment-39716735; this is only the counter and gauge, the timer was already there. Working on the meter still, but this is already good to include I think. 

When merged, I will update the wiki. Let me know if you have any comments/tips etc. Thanks!