Emit Harper-native metric names by default; offer opt-in prom-style mapping for legacy dashboards

The Prometheus exporter emits metrics under names that don't match Harper's native metric names. This divergence has been a recurring source of pain (dashboards built against one source don't work against the other), and currently blocks rollout of the native Grafana datasource plugin to customers who already have Prometheus-exporter-backed dashboards. The platform team has been working around it with an Alloy sidecar — not a scalable solution.

## Goal

**Make the Prometheus exporter emit Harper-native metric names by default.** Offer a configurable name-mangling step to produce Prometheus-style names for users who need legacy dashboard compatibility — an opt-in shim, not the default.

## Why not match prom-style at the source

Renaming Harper-native metrics to Prometheus conventions has caused more headaches than it has solved. Once Harper-native names are the source of truth, all consumers (native Grafana datasource, Prometheus exporter, dashboards, exporters yet to be built) share a single vocabulary.

## Scope

- **Default behavior:** Prometheus exporter outputs metric names matching Harper-native names (with whatever sanitization Prometheus itself requires, e.g. `-` → `_`, but otherwise verbatim).
- **Opt-in compatibility:** A configuration option (e.g. `nameStyle: 'legacy-prom'` or a mapping table) that re-applies the *current* prom-style names so existing customer dashboards keep working unchanged.
- **Migration guidance:** Document the new default, the available mapping, and how to migrate a dashboard.

## Acceptance criteria

- New exporter installs produce Harper-native metric names by default.
- Existing customers can opt into legacy naming with a single config change and keep their dashboards working.
- Documentation calls out the change and the migration path.

## Where this lives

Primary home: `HarperFast/prometheus-exporter` (this issue) — the name-mangling shim and config option live here.

## Out of scope

- Killing the Alloy sidecar — downstream platform concern once dashboards work natively.
- Changes to the [native Grafana datasource](https://github.com/HarperFast/grafana-datasource) — it already speaks Harper-native names, so no changes expected once the exporter aligns.

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Tracked in Jira: [CORE-3044](https://harperdb.atlassian.net/browse/CORE-3044)

🤖 Filed by Claude on behalf of Kris.