metrics: allow metric proto enums to be set with |long| values
metrics: allow metric proto enums to be set with |long| values

This is the simplest change to get enum support in metrics. The types
are sensible since enums are encoded as varints. I've briefly looked at
supporting enums by name via AppendString searching the descriptor for
the enum's name->value conversion, but, I think that to avoid edge cases
we'd need the conversion to also consider the field's enum proto type
(in case the unqualified constant name is ambiguous), and again, it
didn't seem worth adding that logic at this point in time.

Bug: 184265484
Change-Id: Ia85abab569e7290687d097c34aa7b6db50615c55

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto