main: Start heartbeater
main: Start heartbeater

This fix calls Start() on the hearbeater so it can function.

Code comments were also improved to understand how and why the
heartbeater is initialized in main.go

cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools,cloudfoundry-community/stackdriver-tools