Granular Spans for Coprocessors

[andrewmcgivery]

OTEL spans for coprocessors currently appear as one giant span for the entire lifetime of the request. This is confusing when looking at metrics as it is unclear how long the coprocessor is actually taking.

Ideally, each "stage" of the coprocessor would be labeled as a separate span such as coprocessor.SubgraphRequest so the user can differentiate not just the coprocessor but also the different stages of the coprocessor being called.

(raised on behalf of customer, discussed with @BrynCooke)