fix: README

When `GITLAB_TRACING` is set, LabKit automatically creates and configures a tracer:

- **Rails applications**: Railtie automatically inserts Labkit::Tracing::RackMiddleware into the middleware stack

- **Non-Rails applications**: Works automatically with no additional configuration

- **Non-Rails applications**: Tracer is auto-initialized, but `Labkit::Tracing::RackMiddleware` must be manually added to your middleware stack for HTTP request tracing (see [Rack Middleware](#rack-middleware))

Auto-initialization provides:

- Automatic tracer creation with connection string settings (sampler, exporter, service name)

  - **Behavior**: Outputs spans immediately to stdout with no remote export

  - **Benefits**: No external collector required, instant feedback, simple debugging

**HTTPS Enforcement:** For security, non-localhost OTLP connections are automatically upgraded to HTTPS. Localhost addresses (`localhost`, `127.0.0.1`, `::1`) use plain HTTP. For example:

- `otlp://localhost:4318` → `http://localhost:4318`

- `otlp://collector.example.com:4318` → `https://collector.example.com:4318`

Ensure your OTLP collector supports TLS when using non-localhost endpoints.

### Sampling

- **No application code changes required** - just set the environment variable

**Non-Rails Applications (Sinatra, Grape, standalone Ruby):**

- Auto-initialization happens automatically when `require 'gitlab-labkit'` is called, so no manual setup is needed.

- Tracer auto-initialization happens automatically when `require 'gitlab-labkit'` is called.

- However, you must manually add `Labkit::Tracing::RackMiddleware` to your middleware stack for HTTP request tracing. See the [Rack Middleware](#rack-middleware) section for details.

Auto-initialization:

- Creates tracer with connection string settings (sampler, exporter, endpoints)