Enable OpenTelemetry for non-prod environments

What does this MR do and why?

Enable OpenTelemetry for non-prod environments.

See https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/6843+

Tasks

• Follow https://docs.honeycomb.io/getting-data-in/opentelemetry/ruby/ to add gems and initializer
• Restrict initializer to only enable development and staging environments
• Add an GITLAB_ENABLE_OTEL_EXPORTERS env var which is required for enabling the initializer (instead of feature flag or Settings entry, which I'm not sure would be available).
• Add minimal developer documentation page with basic setup/testing instructions.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

• See existing captured tracing data at https://gitlab.com/gitlab-org/remote-development/opentelemetry/development/cwoolley-opentelemetry-development-testing/-/tracing
• Follow new docs to set up on your own project, if desired.