Apollo Tracing (e.g., inline tracing support)

[abernix]

Adds FTV1 support.

A new open telemetry exporter has been added that will convert regular traces to Apollo traces.

A buffer of spans is collected on the server side which will retain spans until the root request span is completed.
Once a request is completed the trace will be reconstructed and sent to Apollo.

Span attributes that are only relevant to Apollo tracing are prefixed with apollo_private. and are filtered out of other APM data.

@glasser Has given some guidance on how we should improve tracing, but this'll be left to followup tickets as this PR is large and has been ongoing for a significant period.

#1728.
#1729.
As an aside, this PR demonstrates that spans can be used for Apollo tracing, and that we could move to a native Otel based solution in future.