.github: issue template for telemetry proposals

.github: issue template for telemetry proposals

This uses the github's newer template syntax.

https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms

See
https://github.com/hyangah/go/issues/new?assignees=&labels=Telemetry-Proposal&projects=hyangah%2F3&template=12-telemetry.yml&title=x%2Ftelemetryconfig%3A+
for demo.

Change-Id: I13105711b1fe97680b505eefa1f0e7082e76e92c
Reviewed-on: https://go-review.googlesource.com/c/go/+/528357
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>