Add a sampling rate to StatsD timer

@bellycard/platform @bellycard/data 

When measuring very fast jobs, the act of timing can have an impact on the method. This pull adds a sampling rate option of when to track. 

```ruby
report_time('metric', sample_rate: 10) do
  # your awesome code gets sampled every 10 milliseconds
end
```

If you don't pass in a `sample_rate`, it defaults to 1. A `sample_rate` of `x` means every `1:x` milliseconds will get logged.