Add Heka exporter to list of third-party exporters.
Add Heka exporter to list of third-party exporters.

pdbogen/docs,chosenken/docs,ddfreyne/prometheus-docs,prometheus/docs,open-falcon/open-falcon.github.io,chosenken/docs,tcolgate/docs,open-falcon/open-falcon.github.io,chosenken/docs,prometheus/docs,tcolgate/docs,pdbogen/docs,vic3lord/docs,vic3lord/docs,tcolgate/docs,open-falcon/open-falcon.github.io,prometheus/docs,ddfreyne/prometheus-docs,du2016/prometheus-docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,vic3lord/docs,Cepave/openfalcon.io,Cepave/openfalcon.io,ddfreyne/prometheus-docs,du2016/prometheus-docs,chosenken/docs,Cepave/openfalcon.io,pdbogen/docs