Disable Prometheus metrics in GraphQL generic tracer

What does this MR do and why?

This tracer creates high-cardinality metrics that do not appear to be used.

This introduces a graphql_generic_tracing_metrics_deactivate feature flag that will deactivate this if enabled.

Relates to #345396 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. On this branch, navigate to a project repository.
2. Use curl to scrape the /-/metrics endpoint of your GDK. You should see hundreds of metrics. For example:

% curl -s https://host.docker.internal:3443/-/metrics | grep graphql_duration_seconds | wc -l
562

3. On this branch, enable the feature flag: Run bin/rails console and Feature.enable(:graphql_generic_tracing_metrics_deactivate).
4. Restart Puma: gdk restart rails
5. Repeat steps 1 to 3. You should see 0 metrics.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.