Support arbitrary field=value labels on custom analytics

## Request

Custom analytics currently supports a fixed set of fields (path, method, type). Extend this to support an arbitrary number of user-defined fields with user-defined value domains, enabling much richer segmentation.

## Proposed

- Allow component authors to attach arbitrary `field=value` labels to analytics events
- Expose these as Grafana labels in the native Harper Grafana plugin so users can query and filter on them
- Investigate Prometheus and Hydrolix support as customer demand warrants

## Use case

Enables filtering analytics by business-domain dimensions (e.g., `region=us-east`, `tenant=acme`, `tier=premium`) rather than only HTTP-level dimensions.

🤖 Filed by Claude on behalf of Kris.