Elastic-apm

* fixed an issue with Celery and the prefork worker pool (444)
* fixed an issue when running uwsgi without a master process (446)
* fixed an issue with gevent/eventlet on Python 3.7 (451, 454)
* introduced `IntervalTimer` and use it instead of `threading.Timer` (452)
* added license header check as pre-commit hook (456)

* fixed an issue with the certificate pinning feature introduced in 4.2.0 (433, 434)
* fixed incompatibility with eventlet introduced in 4.2.0 (435, 436)

* Implemented a new transport queue, which should avoid certain deadlock scenarios (411)
* Implemented server certificate pinning (405)
* Moved context.url to context.http.url for requests/urllib3 spans (393, 394)
* Added support for using route as transaction name in Django 2.2+ (86, 396)
* Added some randomness to time between requests to APM Server (426)
* Fixed an issue with custom user models in Django using non-string usernames (397, 398)
* Fixed an issue with sending kubernetes metadata to the API (401, 402)
* Fixed an issue with parsing /proc/stat in RHEL/centos 6 (406, 407)
* Added copyright header to all files, and a CI check (429)

* Added support for collecting system and process metrics (361)
* Added an OpenTracing bridge (388)
* Added `transaction.sampled` to errors (371)
* Added `transaction.type` to errors (391)
* Added parsing of `/proc/self/cgroup` to capture container meta data (352)
* Added option to configure logging for Flask using a log level (344)
* Added `capture_headers` config option (392)

* implemented de-dotting of tag names and context keys (353)
* wrote a quickfix for the boto3/botocore instrumentation (367)
* fixed an issue with psycopg2 and encoded strings (366)

* fixed another issue in the new v2 transport (351)