Add sampling logic

[quinnmil]

Changes

• Adds Sampler class which handles:
• Loading sampling configuration
• Finding the matching job/controller pattern for a given operation name. Update: This has been simplified to just use a startswith to be consistent with how the current ignore_path logic works.
• get sampling rate for matched job/controller, make sampling decision based on priority order.
• Updates Config module to accept new configuration options: sample_endpoint_rate and sample_job_rate which work as a endpoint/job catchall, with the fallback value still being the global sample_rate.