Add Promtail service to Docker Compose and update Loki configuration

- Introduced a new Promtail service in `docker-compose.observability.yml` for log collection.
- Configured Promtail with necessary volumes and command for log processing.
- Updated `loki-config.yml` to include `allow_structured_metadata` setting for enhanced metadata handling.

docs/en/guides/observability.md

@@ -0,0 +1,89 @@
+# Observability Stack Guide
+
+This guide explains how to use the observability stack (Grafana, Prometheus, Loki, Promtail) included in the infrastructure.
+
+## Architecture Overview
+
+The stack consists of the following components:
+
+-   **Prometheus**: Collects metrics from services.
+-   **Loki**: Collects logs.
+-   **Promtail**: Scrapes logs from Docker containers and pushes them to Loki.
+-   **Grafana**: Visualization dashboard for metrics (from Prometheus) and logs (from Loki).
+
+## Getting Started
+
+### Prerequisites
+
+-   Docker and Docker Compose installed.
+-   Existing `microservices-network` (created by the main application stack or manually).
+
+### Starting the Stack
+
+You can easily start the stack using the provided script:
+
+```bash
+./scripts/observability/start.sh
+```
+
+Or manually:
+
+```bash
+# Ensure network exists
+docker network create microservices-network || true
+
+cd infra/observability
+docker-compose -f docker-compose.observability.yml up -d
+```
+
+Check if all containers are running:
+
+```bash
+docker ps
+```
+
+You should see `grafana`, `prometheus`, `loki`, and `promtail`.
+
+## Accessing Services
+
+| Service | URL | Credentials (if applicable) | Description |
+| :--- | :--- | :--- | :--- |
+| **Grafana** | [http://localhost:3001](http://localhost:3001) | `admin` / `admin` | Main dashboard for visualization. |
+| **Prometheus** | [http://localhost:9090](http://localhost:9090) | N/A | Raw metrics and target status. |
+| **Loki** | [http://localhost:3100](http://localhost:3100) | N/A | Log aggregation API (no UI). |
+
+## Using Grafana
+
+1.  **Login**: Access [http://localhost:3001](http://localhost:3001) and login with `admin`/`admin`.
+2.  **Explore Data**:
+    -   Go to **Explore** (compass icon) in the sidebar.
+    -   Select **Loki** from the datasource dropdown to search logs.
+    -   Select **Prometheus** from the datasource dropdown to query metrics.
+
+### Viewing Logs (Loki)
+
+In the **Explore** view with **Loki** selected:
+
+1.  Click **Label browser**.
+2.  Select a label, e.g., `container`.
+3.  Choose a specific container (e.g., `auth-service` or `traefik`).
+4.  Click **Show logs**.
+
+You can also write LogQL queries manually, for example:
+
+```logql
+{container="auth-service"}
+```
+
+### Viewing Metrics (Prometheus)
+
+In the **Explore** view with **Prometheus** selected:
+
+1.  Type a metric name in the query field (e.g., `up`, `container_memory_usage_bytes`).
+2.  Click **Run query**.
+
+## Configuration
+
+-   **Prometheus**: Rules and targets are configured in `infra/observability/prometheus/prometheus.yml`.
+-   **Promtail**: Log scraping rules are configured in `infra/observability/promtail/promtail-config.yml`.
+-   **Grafana**: Datasources and dashboards provisioning are in `infra/observability/grafana/`.