Set metric API

[waj]

Implement StatsD like sets metric API (https://github.com/etsy/statsd/blob/master/docs/metric_types.md#sets)

Telemetry.set_add(:channels, {project: 3, type: :smpp})

[juanedi]

Changed the API to:

InsteddTelemetry.set_add(:channels, {project_id: 1}, :smpp)

• channels is a tag that denotes what it is we are counting.
• {project_id: 1} is the key of the set. more attributes can be added and they will all be used to identify the set.
• :smpp is the element we are adding to the set.