nozzle: tweaks to error instrumentation
nozzle: tweaks to error instrumentation

- use goroutines to increment heartbeater. it can block in a scenario
  where metric/log handlers can't flush.
- drop nil errs, this is a bandaid on an unknown issue.

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