feat: normalize prometheus metric name

Ensure the Prometheus metric name are respecting Prometheus convention.

mqtt_exporter/main.py

@@ -9,7 +9,7 @@
 import sys
 
 import paho.mqtt.client as mqtt
-from prometheus_client import Counter, Gauge, start_http_server
+from prometheus_client import Counter, Gauge, metrics, start_http_server
 
 from mqtt_exporter import settings
 
@@ -61,13 +61,33 @@ def subscribe(client, _, __, result_code, *args):
         LOG.error("MQTT %s", mqtt.connack_string(result_code))
 
 
+def _normalize_prometheus_metric_name(prom_metric_name):
+    """Transform an invalid prometheus metric to a valid one.
+
+    https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
+    """
+    if metrics.METRIC_NAME_RE.match(prom_metric_name):
+        return prom_metric_name
+
+    # clean invalid characted
+    prom_metric_name = re.sub(r"[^a-zA-Z0-9_:]", "", prom_metric_name)
+
+    # ensure to start with valid character
+    if not re.match(r"^[a-zA-Z_:]", prom_metric_name):
+        prom_metric_name = ":" + prom_metric_name
+
+    return prom_metric_name
+
+
 def _create_prometheus_metric(prom_metric_name):
     """Create Prometheus metric if does not exist."""
     if not prom_metrics.get(prom_metric_name):
         labels = [settings.TOPIC_LABEL]
         if settings.MQTT_EXPOSE_CLIENT_ID:
             labels.append("client_id")
 
+        prom_metric_name = _normalize_prometheus_metric_name(prom_metric_name)
+
         try:
             prom_metrics[prom_metric_name] = Gauge(
                 prom_metric_name, "metric generated from MQTT message.", labels

tests/unit/__init__.py

@@ -0,0 +1 @@
+"""Unit tests."""

tests/unit/test_normalize_prometheus_name.py

@@ -0,0 +1,15 @@
+"""Tests of Prometheus normalization metrics."""
+from mqtt_exporter.main import _normalize_prometheus_metric_name
+
+
+def test_normalize_prometheus_metric_name():
+    """Test _normalize_prometheus_metric_name."""
+    tests = {
+        "1234invalid": ":1234invalid",
+        "valid1234": "valid1234",
+        "_this_is_valid": "_this_is_valid",
+        "not_so_valid%_name": "not_so_valid_name",
+    }
+
+    for candidate, wanted in tests.items():
+        assert _normalize_prometheus_metric_name(candidate) == wanted