atc: wire tracing
atc: wire tracing

this commit makes the tracing functionality available by allowing operators to
specify `--tracing-*` flags to the `concourse web` command to leverage the
tracing infrastructure put in place.

with it, support for submitting the traces to a jaeger collector is configured
like so:

	--tracing-jaeger-endpoint=<>
	--tracing-jaeger-service=<>

and, stackdriver:

	--tracing-stackdriver-projectid=<>

ps.: these can also be configured via environment variables
pps.: only 1 trace exporter can be initialized

refs:
  - jaeger: https://www.jaegertracing.io/
  - stackdriver trace: https://cloud.google.com/trace/

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse