Histograms, Scalars, and Events are cheap but not free. Expect performance
impacts if you use them in tight loops or hot paths.
MozReview-Commit-ID: 2nrdHXrbN9i
Having "main" be the default value is a very sensible choice, but one that is a
little opaque if you aren't into reading Telemetry code.
Just require a value here, and this will encourage correct use as it can be
caught and policed in data-review.
MozReview-Commit-ID: ASaWGuPAc9a