[opentelemetry-integration] chore: add prometheus receiver debuging d…

…ocs (#487)

otel-integration/k8s-helm/README.md

@@ -541,12 +541,12 @@ Once you enable the Span Metrics preset, the errorTracking configuration will au
 
 This is how you can disable the errorTracking option:
 
-```
+```yaml
 presets:
-    spanMetrics:
-      enabled: true
-      errorTracking:
-        enabled: false
+  spanMetrics:
+    enabled: true
+    errorTracking:
+      enabled: false
 ```
 
 Note: errorTracking only works with OpenTelemetry SDKs that support OpenTelemetry Semantic conventions above v1.21.0. If you are using older versions, you might need to transform some attributes, such as:
@@ -557,7 +557,7 @@ http.status_code => http.response.status_code
 
 To do that, you can add the following configuration:
 
-```
+```yaml
 presets:
   spanMetrics:
      enabled: true
@@ -575,12 +575,12 @@ This is needed to enable the [Database Monitoring](https://coralogix.com/docs/us
 
 This is how you can disable the dbMetrics option:
 
-```
+```yaml
 presets:
-    spanMetrics:
-      enabled: true
-      dbMetrics:
-        enabled: false
+  spanMetrics:
+    enabled: true
+    dbMetrics:
+      enabled: false
 ```
 
 Note: DbMetrics only works with OpenTelemetry SDKs that support OpenTelemetry Semantic conventions v1.26.0. If you are using older versions, you might need to transform some attributes, such as:
@@ -594,7 +594,7 @@ db.cassandra.table => db.collection.name
 
 To do that, you can add the following configuration:
 
-```
+```yaml
     spanMetrics:
       enabled: false
       dbMetrics:
@@ -742,6 +742,85 @@ service:
 
 This adds more metrics around exporter latency and various processors metrics.
 
+### Prometheus Receiver
+
+If you are missing metrics collected by Prometheus receiver make sure to check Collector logs.
+
+The Prometheus receiver typically logs `Failed to scrape Prometheus endpoint` errors with target information when it fails to collect the application metrics.
+
+For example:
+
+```
+message_obj:{
+level:warn
+ts:2024-12-13T08:19:17.809Z
+caller:internal/transaction.go:129
+msg:Failed to scrape Prometheus endpoint
+kind:receiver
+name:prometheus
+data_type:metrics
+scrape_timestamp:1734077957789
+target_labels:{__name__="up", container="main", endpoint="4001",  namespace="namespace", pod="pod-name"}
+}
+```
+
+The generic error, doesn't tell you much. To get more details, you will need to enable debug logs inside the Collector:
+
+```yaml
+global:
+  logLevel: "debug"
+```
+
+Then you will start seeing the actual metric and error in Collector logs, this will help you troubleshoot it further.
+
+**Common Errors**
+
+`invalid sample: non-unique label names` - Metric contains non-unique label names. For example:
+
+```yaml
+metric{label1="value1",label1="value2"}
+```
+
+This is not allowed in Prometheus / [OpenMetrics](https://github.com/prometheus/OpenMetrics/blob/main/specification/OpenMetrics.md#labelset), but some libraries produce such labels. It is best to fix the application or library. But as a workaround, you can fix it with [metric_relabel_configs](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#metric_relabel_configs), which gets executed before the metric is ingested.
+
+For example, you drop the `label1` metric:
+
+```yaml
+metric_relabel_configs:
+  - action: labeldrop 
+    regex: 'label1'
+```
+
+Alternatively, you can replace the metric with itself, leaving only single label:
+
+```yaml
+metric_relabel_configs:
+  - action: replace
+    source_labels: ['label1']
+    target_label: label1
+```
+
+`'le' label on histogram metric is missing or empty.` Histogram metric contains multiple types. Typically, the metric library produces invalid metrics that are both a histogram and a summary, which is not allowed in Prometheus / OpenMetrics. For example:
+
+```
+# HELP http_server_requests_seconds  
+# TYPE http_server_requests_seconds histogram
+http_server_requests_seconds_bucket{le="0.025",} 1
+http_server_requests_seconds_count{} 15.0
+http_server_requests_seconds_sum{} 0.20938292
+...
+http_server_requests_seconds{quantile="0.999",} 0.0
+```
+
+It is best to fix the application or library to produce just histogram. But as a workaround, you can fix it with `metric_relabel_config`. The following example will drop metrics with quantile label:
+
+```yaml
+metric_relabel_configs
+ - sourceLabels: [__name__, quantile]
+   regex: http_server_requests_seconds;.*
+   action: drop
+```
+
 ## Traces
 
 OpenTelemetry Collector has an ability to send it's own traces using OTLP exporter. You can send the traces to OTLP server running on the same OpenTelemetry Collector, so it goes through configured pipelines. For example: