[Bug]: Critical! MCP operations are not counted in DAB custom metrics

MCP tool invocations (describe_entities, read_records, create_record, update_record, delete_record, aggregate_records) do not appear in any of the 4 DAB custom OTEL metrics (`active_requests`, `total_requests`, `total_errors`, `request_duration`).

## Expected

MCP operations should be tracked with `api_type: MCP` alongside REST and GraphQL, providing uniform observability across all three API surfaces.

```
NumberDataPoints #N
Data point attributes:
-> api_type: Str(MCP)
-> endpoint: Str(read_records)
-> method: Str(Post)
-> status_code: Str(OK)
Value: 1
```

## Actual

The `api_type` metric dimension only contains `REST` (22 data points) and `GraphQL` (6 data points). Zero data points exist with `api_type: MCP`, despite 6 successful MCP operations being executed during testing.