Add prometheus metrics

[EvilLord666]

For collecting metrics of performance we should add go prometheus. Before this task we should define what type of metrics we should collect. Metric would be listed here after 0.9.2 is finished

We should have 2 sets of metrics:

1. SRE (site reliability) Golden metrics:
   • Latency - time of request processing (type Histogram) with excluded REST API admin methods call
   • Users (Traffic) - number of active users == number of sessions (type Gauge)
   • Errors - number of 500 errors in Http Response (probably Histogram of 2 values error && no error, consider errors is 500 only, other responses are non error response)
   • Saturation - resource utilization (CPU, memory in Mb and number of goroutines)
2. Additional metrics (think here about what should we have here)
   • probably sent e-mails count (Gauge)