[agent_metrics] add metrics for num_metrics and num_events

[cberry777]

What does this PR do?

Adds additional metrics for num_metrics and num_events to agent_metrics

Motivation

It is very important to monitor the number of metrics and events emitted from each agent. It allows us to 1) keep track of the total number of metrics sent to Datadog (to monitor billing), and 2) locate rogue agents emitting above some threshold.

Testing Guidelines

A test is provided: /tests/checks/mock/test_agent_metrics.py (# test_num_metrics)

Additional Notes

An optional switch is provided (in the init_config) that allows one to log the number of metrics and events for each collection run.

[remh]

Thanks @cberry777
FYI, i don't think it will be really useful billing wise as:

• This just counts metrics coming from checks.d and not old style checks (most of system metrics)
• It doesn't count dogstatsd metrics
• It doesn't differentiate between integration metrics and custom metrics

However, it might be useful to track that so we'll get it merged for our 5.11 release.

Can you have a look at the failing tests please ?

[cberry777]

Again. Tests are failing that have nothing to do with this code.
I think that the test suite is unstable??

FAIL: Support SNMP scalar objects
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/DataDog/dd-agent/tests/checks/integration/test_snmp.py", line 267, in test_scalar
    self.assertMetric(metric_name, tags=self.CHECK_TAGS, count=1)
  File "/home/travis/build/DataDog/dd-agent/tests/checks/common.py", line 350, in assertMetric
    self._candidates_size_assert(candidates, count=count, at_least=at_least)
  File "/home/travis/build/DataDog/dd-agent/tests/checks/common.py", line 320, in _candidates_size_assert
    "Needed exactly %d candidates, got %d" % (count, len(candidates))

All tests pass when I run "rake"

---

Ran 176 tests in 20.422s

OK (SKIP=1)
Cleaning up

[cberry777]

Is there a way to "re-fire" the test suite??
(without forcing a bogus commit)

[masci]

It was a flaky test, all green now.

[gmmeyer]

Hey @cberry777! Thanks a lot for your contribution.

I think I missed this one when we went through our SDK move. This should be moved to our Integrations Core repo and closed here. I looked it over and don't see anything standout that needs to be changed. If you move it I see no reason it couldn't be merged easily!