Describe what happened:
When datadog agent receives SIGTERM, it won't flush the metrics before it dies.

Describe what you expected:
When it receives SIGTERM, I would expect Datadog Agent flushes out all the currently aggregated metrics before it gracefully shuts down.

Steps to reproduce the issue:
Step 1: Start the Datadog Agent
Step 2: Emit a metric to Datadog Agent
Step 3: Send a SIGTERM signal to Datadog Agent before 15 seconds flush interval ends.

Metrics emitted at step 2 will simply lost and not available.

Additional environment details (Operating System, Cloud provider, etc):