Telemetry experiment activity added to FHR payload

[Gregory Szorc]

Sometime in the next week or so the first Telemetry Experiment will be 
rolled out to the Nightly channel. When it it, FHR will start emitting a 
new per-day measurement describing the most recent active experiment for 
that day.

See 
https://hg.mozilla.org/integration/fx-team/diff/c542b74bd763/services/healthreport/docs/dataformat.rst 
for the measurement description.