.github: improve formatting of the telemetry proposal template

Tweak the new telemetry proposal template added in CL 583496:
 - Shorten the description, as it is formatted on one conspicuously long
   line in the template picker.
 - Use folded style for label descriptions, as their line breaks cause
   the resulting paragraph to flow awkwardly.

Change-Id: I3089ac0717646e153765548d4bebd8d4751933b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/583916
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>