OPS-5392 add datadog event support

[nirajdewani]

Description

I'd like to add support for Datadog Events. Pasting context from OPS-5392:

Currently back pressure sends metrics to Datadog (https://app.datadoghq.com/dashboard/qnx-z63-3di/backpressure-monitoring). We should send datadog events instead (or as well): https://docs.datadoghq.com/service_management/events/ This would enable us to do things like:

• Trigger DD monitors for alerting purposes (eg. notifying slack / PagerDuty)
• overlay events on top of dashboard widgets
• add tags to events to support analysis / grouping

(Potential) To Do

Added a unit test; please let me know if/how I can test this better.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 80.01%. Comparing base (7b49dc5) to head (f2deaff).

:exclamation: Current head f2deaff differs from pull request most recent head 4ed7273. Consider uploading reports for the commit 4ed7273 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #70399       +/-   ##
===========================================
+ Coverage   53.63%   80.01%   +26.38%     
===========================================
  Files        6501     6505        +4     
  Lines      290588   290811      +223     
  Branches    50105    50124       +19     
===========================================
+ Hits       155851   232695    +76844     
+ Misses     133763    57679    -76084     
+ Partials      974      437      -537     

Files	Coverage Δ
src/sentry/metrics/base.py	96.00% <100.00%> (+12.66%)	:arrow_up:
src/sentry/metrics/composite_experimental.py	78.26% <50.00%> (+35.07%)	:arrow_up:
src/sentry/metrics/datadog.py	86.44% <87.50%> (+55.06%)	:arrow_up:
src/sentry/metrics/dummy.py	92.30% <50.00%> (-7.70%)	:arrow_down:
src/sentry/metrics/minimetrics.py	97.46% <50.00%> (+55.90%)	:arrow_up:
src/sentry/metrics/statsd.py	86.36% <50.00%> (+31.36%)	:arrow_up:
src/sentry/metrics/middleware.py	94.87% <20.00%> (+19.52%)	:arrow_up:
src/sentry/utils/metrics.py	78.44% <16.66%> (-3.37%)	:arrow_down:

... and 3333 files with indirect coverage changes