Forward only key fields to peers

[VinozzZ]

We will extend the existing peer span forwarding system to include forwarding a “proxy span” to its decider node. This proxy span will include:

• Key fields necessary for trace decisions (this can be empty if no key fields are present).
• A marker that differentiates proxy spans from normal spans.

Conditions for Forwarding a Proxy Span:

• ForceTraceLocality is set to false.
• A new span arrives, even if it does not contain any key fields.

Ignoring Proxy Spans:
When sending spans to Honeycomb, Refinery should ignore proxy spans by checking the marker to ensure they are not forwarded.

[MikeGoldsmith]

Requires the following issue to be resolved first:

• #1325