Emit subgraph per-field metrics

[joemccall86]

Is your feature request related to a problem? Please describe.

We're using progressive override to migrate fields and objects from a legacy subgraph to a new one. We'd like to track the actual rollout and visualize it in a Grafana dashboard.

Describe the solution you'd like

We can currently emit custom instruments with the GraphQL field type and field name, and type name. I'd like to know the subgraph that resolved these fields. From there I can build out the dashboard.

Describe alternatives you've considered

• Parsing the request/response body in a coprocessor (too complex due to the possibility of nesting)
• Viewing the raw query plan in the coprocessor (again, the data wasn't easily extractable)
• Emitting metrics in the individual subgraphs (wouldn't work, because the subgraphs are unaware of individual field-level overrides)
• Retrieving the field-level metrics from the subgraph (would require compiling protobuf to create go-compatible structs. Comment at the top of the .proto file indicated that we should reach out for support before building on top of it.)

Additional context

We're not just talking about the percent(x) labels, but custom overrides for things like LaunchDarkly integration.