Aggregated tracer health metrics

[delner]

To monitor the health and performance of the tracer, this pull request adds health metrics to the tracer and other core components, which are sent as Statsd metrics under the datadog.tracer.* prefix. These metrics can then be graphed to evaluate the health, stability and impact of the trace library within a Ruby application.

By default, these metrics are enabled, if dogstatsd-ruby >= 3.3.0 is available. They can be disabled by setting the DD_DEBUG_HEALTH_METRICS_ENABLED ENV var to 0.

List of metrics added with this pull request:

datadog.tracer.api.errors
datadog.tracer.api.requests
datadog.tracer.api.responses
datadog.tracer.queue.accepted
datadog.tracer.queue.accepted_lengths
datadog.tracer.queue.dropped
datadog.tracer.queue.length
datadog.tracer.queue.max_length
datadog.tracer.queue.spans
datadog.tracer.traces.filtered # Added but not implemented
datadog.tracer.writer.cpu_time # Added but not implemented

This pull request is based upon #838 and changes its implementation to aggregate queue metrics and reduce the number of Statsd events sent.