Ingest existing Prometheus metrics via remote write

[mehditow]

Problem:

Users already running Prometheus have metrics they'd want to surface in DashFrog without re-instrumenting.

Proposed design:

1. In DashFrog UI, click "Create metric" → select "Prometheus" as source
2. Configure:
   • Select which labels to include
   • Map an existing label to tenant (e.g. customer_id → tenant)
3. UI generates a remote write config snippet to add to your Prometheus setup

Open questions:

• Histogram compatibility: DashFrog uses native histograms — if user sends classical histograms (_bucket/_sum/_count), do we convert, reject, or degrade gracefully?