[Bug]: All 4 custom DAB metrics have empty descriptions

The `DataApiBuilder.Metrics` instrumentation scope defines 4 metrics (`active_requests`, `total_requests`, `total_errors`, `request_duration`), but none of them include a `Description` field. This violates OTEL best practices and makes metrics harder to understand in dashboards and alerting tools.

## Expected

Each metric should include a human-readable description, per the [OTEL metric semantic conventions](https://opentelemetry.io/docs/specs/otel/metrics/api/#instrument):

```
Metric: active_requests
Description: "Number of in-flight API requests currently being processed"

Metric: total_requests
Description: "Cumulative count of completed API requests"

Metric: total_errors
Description: "Cumulative count of failed API requests"

Metric: request_duration
Description: "Duration of API request processing in milliseconds"
```

## Actual

All 4 metrics export with blank descriptions:

```
Descriptor:
-> Name: active_requests
-> Description:
-> Unit:
```

The `Unit` field is also empty for `active_requests`, `total_requests`, and `total_errors` (only `request_duration` correctly sets `Unit: ms`).

Durchsuche das Repository nach dem Instrumentierungs-Scope DataApiBuilder.Metrics und den vier Metriknamen. Prüfe, wie ihre Deskriptoren konfiguriert sind, füge die menschenlesbaren Beschreibungen aus dem Issue hinzu und behebe die fehlenden Einheiten, die für die Zählmetriken vermerkt wurden. Führe die relevanten Metrik- oder Instrumentierungstests aus, sofern vorhanden, und verifiziere, dass die exportierten Deskriptoren befüllt sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.