Rename ingestor-prefixed connection metrics shared with collector

## Problem

Two Prometheus metrics carry the `ingestor` subsystem in their names but are populated by both the ingestor and the collector:

- `adxmon_ingestor_active_connections` (gauge) — Go var `metrics.IngestorActiveConnections`
- `adxmon_ingestor_dropped_connections_total` (counter) — Go var `metrics.IngestorDroppedConnectionsTotal`

Both are incremented in `pkg/limiter/listener.go`, which is wired up by `pkg/http.NewServer`. That HTTP server is used by the collector as well as the ingestor, so both processes emit these metrics. The names are misleading and the collector's `Status` log line scans for them by their ingestor-prefixed names (see `collector/metrics/service.go`).

## Proposal

Rename the metrics and variables to something process-neutral, e.g.:

- `adxmon_server_active_connections`
- `adxmon_server_dropped_connections_total`

Update the Go variable names (`ServerActiveConnections`, `ServerDroppedConnectionsTotal`) and the substring matches in `collector/metrics/service.go` and `ingestor/metrics/service.go` accordingly.

## Compatibility

This is a breaking change for any dashboards or alerts that reference the existing metric names. We should:

1. Audit dashboards under `dashboards/` and `build/k8s/dashboards/` for references.
2. Coordinate the rename with any external consumers / Kusto ingestion mappings if applicable.

## Context

Discovered while cleaning up the per-process Status log lines (branch `cleanup/remove-dead-ingestor-samples-logging`). Decided to leave the rename for a separate change since the existing metric is populated correctly — only the name is misleading.

Start with pkg/limiter/listener.go and the metrics variables, then trace the matching logic in collector/metrics/service.go and ingestor/metrics/service.go. Audit dashboards under dashboards/ and build/k8s/dashboards/ for the existing metric names, and check whether external consumers or Kusto ingestion mappings are affected. Done means the process-neutral names and variables are used consistently and the dashboard and compatibility impact is addressed.

Written by the indexing model from the issue text.