Add backend event tracking to Observability pages

What does this MR do and why?

Adds backend internal event tracking to the Observability feature's pages, which previously had zero instrumentation anywhere in the flow (setup → access request → settings → dashboard).

Six new events, each with a matching unique-user (7d/28d) metric:

Event	Where	Trigger
visit_group_observability_dashboard	Groups::ObservabilityController#show	Successful render branch only
visit_project_observability_dashboard	Projects::ObservabilityController#show	Successful render branch only (not on 404 / redirect-to-setup)
visit_observability_setup_page	Observability::SetupActions#show (shared by group + project setup controllers)	Every successful render
create_observability_access_request	Observability::AccessRequestActions#create (shared by group + project access-request controllers)	Success branch only
visit_o11y_service_settings_page	Groups::Observability::O11yServiceSettingsController#edit	Every page view
update_o11y_service_settings	Groups::Observability::O11yServiceSettingsController#update	Success branch only

The dashboard and setup events carry a label additional property (the observability sub-path, or the group/project context) to distinguish where in the flow the event fired.

Instrumentation uses Gitlab::InternalEventsTracking#track_internal_event directly in each controller/shared concern.

Screenshots or screen recordings

N/A — backend-only tracking, no UI changes.

How to set up and validate locally

1. Enable the observability feature flags (observability_sass_features for a group, or observability_saas_features_user_namespace for a personal namespace).

2. Visit a group or project Observability dashboard, the setup page, and the group settings page (/groups/<group>/-/observability/o11y_service_settings/edit).

3. Check Gitlab::InternalEvents is invoked via the Rails console or by running the added specs:

   bundle exec rspec \
     spec/requests/groups/observability_controller_spec.rb \
     spec/requests/projects/observability_controller_spec.rb \
     spec/requests/groups/observability/o11y_service_settings_spec.rb \
     spec/requests/groups/observability/setup_spec.rb \
     spec/requests/projects/observability/setup_spec.rb \
     spec/requests/groups/observability/access_requests_spec.rb \
     spec/requests/projects/observability/access_requests_spec.rb

All 129 examples pass, including negative tests asserting events are not fired on 404/redirect/failure branches.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

Related issue: #608071