Remote sampling specification for proto layout

[annanay25]

Per @jmacd's comment here - open-telemetry/opentelemetry-collector#432 (comment) - since remote sampling configuration (for both metrics and traces) is on the roadmap for OpenTelemetry, creating this issue as a placeholder for discussion on the specification for proto layout.

[bogdandrutu]

In OpenCensus we had something but was not fully backed and used, but you can take a look at the proto repo config package.

[yurishkuro]

I would separate support for the existing Jaeger format (if desired to expose it) from the design of a new format. We have started building support for user-configured adhoc sampling rules in Jaeger, which require much more generic remote config than what we have today.

[Oberon00]

Hey, I'm not sure I understand what this issue really is about? What's that about "layout" in the title? Maybe a better title can be found?