docs: add testing with grafana otel docs

Author: Leandros

TESTING.md

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+## Testing with Grafana OTEL LGTM Docker container
+
+The Grafana OTEL LGTM Docker container is a container that bundles Grafana,
+Prometheus, Loki, Tempo and a OTEL collector into a single image for testing,
+development and demonstration purposes. We can use it to test `LiteMon` with
+`Alloy`.
+
+Below the process is described:
+
+
+### 1. Spin up Grafana & Prometheus
+
+Run Grafana and Prometheus using the described container image.
+
+```bash
+docker run \
+    -p 3000:3000 \
+    -p 4317:4317 \
+    -p 4318:4318 \
+    -p 3100:3100 \
+    -p 9090:9090 \
+    -p 4040:4040 \
+    -p 3200:3200 \
+    -ti \
+    grafana/otel-lgtm
+```
+
+
+### 2. Download and run Alloy
+
+Alloy is the scraper that scrapes `LiteMon` and pushes metrics to Prometheus.
+
+```bash
+# Download & install alloy (e.g., on RHEL)
+wget https://github.com/grafana/alloy/releases/download/v1.9.1/alloy-1.9.1-1.amd64.rpm
+sudo yum installlocal ./alloy-1.9.1-1.amd64.rpm
+
+# Run alloy with testing config
+./alloy-linux-amd64 run ./testing/config.alloy
+```
+
+
+### 3. Run LiteMon
+
+Last step is to run LiteMon to allow it to be scraped by Alloy.
+
+```bash
+cargo run
+```
+
+
+### 4. Open Grafana
+
+In your web browser navigate to [localhost:3000](http://localhost) to open the
+Grafana Web UI.

testing/config.alloy

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+logging {
+    level = "debug"
+}
+
+livedebugging {
+    enabled = true
+}
+
+prometheus.remote_write "EXAMPLE" {
+  endpoint {
+    url = "http://localhost:9090/api/v1/write"
+  }
+}
+
+// https://grafana.com/docs/alloy/latest/reference/components/discovery/discovery.relabel/
+discovery.relabel "integrations_litemon_exporter" {
+  targets = [
+    // Optionally, update to your listen address and port.
+    {"__address__" = "localhost:9774"},
+  ]
+
+  rule {
+    target_label = "instance"
+    replacement  = constants.hostname
+  }
+
+  rule {
+    target_label = "job"
+    replacement  = "integrations/litemon"
+  }
+}
+
+// https://grafana.com/docs/alloy/latest/reference/components/prometheus/prometheus.scrape/
+prometheus.scrape "litemon_exporter" {
+  job_name        = "integrations/litemon"
+  targets         = discovery.relabel.integrations_litemon_exporter.output
+  scheme          = "http"
+  scrape_interval = "10s"
+  metrics_path    = "/metrics"
+
+  // Update endpoint to your prometheus ingest.
+  forward_to = [prometheus.remote_write.EXAMPLE.receiver]
+}