Allow specifying the alertlib kind, so we don't hard-code to GAUGE.
Allow specifying the alertlib kind, so we don't hard-code to GAUGE.

Summary:
I want to start sending cumulative stats for some of the ec2 machines,
e.g. log sizes.

Custom metrics cannot have metric kind `delta`.

According to
https://cloud.google.com/monitoring/api/v3/metrics#intro-time-series

Test Plan:
tests/timeout_test.py
tests/alertlib_test.py

(none new)

Reviewers: dhruv, alexanderforsyth

Reviewed By: alexanderforsyth

Differential Revision: https://phabricator.khanacademy.org/D28625

Khan/alertlib