feat: add SLI module for Service Level Indicators

feat: add SLI module for Service Level Indicators

Move the Gitlab::Metrics::Sli module from GitLab-rails into labkit-ruby to enable reuse across multiple GitLab services. The module provides Apdex and ErrorRate SLI types for observability.

This also renames the module to ApplicationSli to use consistent naming as we do in the docs.

The implementation uses Labkit::Metrics::Client for Prometheus counters while maintaining the 'gitlab_sli' counter prefix for consistency.

This is an exact copy of the class and tests from GitLab-rails

For #52 (closed)