[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`).