Add CustomStringConvertible to TestMetrics

[kukushechkin]

Currently, when testing, there is not way to print out the counter to see what is in it.

Motivation:

Having a way to print a human-readable representation of the TestMetrics is useful during the debugging.

Modifications:

Conform TestMetrics to CustomStringConvertible and implement human-readable representation of its content.

Result:

TestMetrics object can now be printed and produces a string representation looking like this:

// TestMetrics:
// Counters:
//   TestCounter(requests, dimensions: [("method", "GET")], values: [10, 5], total: 15)
// Recorders:
//   TestRecorder(response.size, dimensions: [], aggregate: true, values: [1024.0])
// Timers:
//   TestTimer(latency, dimensions: [], valuesNanos: [1000000])