feat: Add prometheus.echo component for local metrics inspection

.github/ISSUE_TEMPLATE/blank.yaml

@@ -144,6 +144,7 @@ body:
       - otelcol.receiver.vcenter
       - otelcol.receiver.zipkin
       - otelcol.storage.file
+      - prometheus.echo
       - prometheus.enrich
       - prometheus.exporter.apache
       - prometheus.exporter.azure

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -144,6 +144,7 @@ body:
       - otelcol.receiver.vcenter
       - otelcol.receiver.zipkin
       - otelcol.storage.file
+      - prometheus.echo
       - prometheus.enrich
       - prometheus.exporter.apache
       - prometheus.exporter.azure

.github/ISSUE_TEMPLATE/docs.yaml

@@ -147,6 +147,7 @@ body:
       - otelcol.receiver.vcenter
       - otelcol.receiver.zipkin
       - otelcol.storage.file
+      - prometheus.echo
       - prometheus.enrich
       - prometheus.exporter.apache
       - prometheus.exporter.azure

.github/ISSUE_TEMPLATE/feature_request.yaml

@@ -144,6 +144,7 @@ body:
       - otelcol.receiver.vcenter
       - otelcol.receiver.zipkin
       - otelcol.storage.file
+      - prometheus.echo
       - prometheus.enrich
       - prometheus.exporter.apache
       - prometheus.exporter.azure

.github/ISSUE_TEMPLATE/proposal.yaml

@@ -144,6 +144,7 @@ body:
       - otelcol.receiver.vcenter
       - otelcol.receiver.zipkin
       - otelcol.storage.file
+      - prometheus.echo
       - prometheus.enrich
       - prometheus.exporter.apache
       - prometheus.exporter.azure

CHANGELOG.md

@@ -24,6 +24,8 @@ Main (unreleased)
 
 - Add htpasswd file based authentication for `otelcol.auth.basic` (@pkarakal)
 
+- Add `prometheus.echo` component for local inspection of Prometheus metrics. The component writes received metrics to stdout in Prometheus exposition format, enabling easier debugging and testing of metrics flow. (@iamrajiv)
+
 ### Enhancements
 
 - update promtail converter to use `file_match` block for `loki.source.file` instead of going through `local.file_match`. (@kalleep)

docs/sources/reference/compatibility/_index.md

@@ -187,6 +187,7 @@ The following components, grouped by namespace, _export_ Prometheus `MetricsRece
 {{< /collapse >}}
 
 {{< collapse title="prometheus" >}}
+- [prometheus.echo](../components/prometheus/prometheus.echo)
 - [prometheus.enrich](../components/prometheus/prometheus.enrich)
 - [prometheus.relabel](../components/prometheus/prometheus.relabel)
 - [prometheus.remote_write](../components/prometheus/prometheus.remote_write)

docs/sources/reference/components/prometheus/prometheus.echo.md

@@ -0,0 +1,122 @@
+---
+canonical: https://grafana.com/docs/alloy/latest/reference/components/prometheus/prometheus.echo/
+description: Learn about prometheus.echo
+labels:
+  stage: general-availability
+  products:
+    - oss
+title: prometheus.echo
+---
+
+# `prometheus.echo`
+
+The `prometheus.echo` component receives Prometheus metrics and writes them to stdout in Prometheus exposition format.
+This component is useful for debugging and testing the flow of metrics through a pipeline, allowing you to see exactly what metrics are being received at a particular point in your configuration.
+
+## Usage
+
+```alloy
+prometheus.echo "<LABEL>" {
+}
+```
+
+## Arguments
+
+You can use the following arguments with `prometheus.echo`:
+
+| Name     | Type     | Description                                                     | Default | Required |
+| -------- | -------- | --------------------------------------------------------------- | ------- | -------- |
+| `format` | `string` | The output format for metrics. Must be `text` or `openmetrics`. | `text`  | no       |
+
+The `format` argument controls how metrics are encoded when written to stdout:
+
+* `text` - Uses the Prometheus text exposition format (default).
+* `openmetrics` - Uses the OpenMetrics text format.
+
+## Blocks
+
+The `prometheus.echo` component doesn't support any blocks. You can configure this component with arguments.
+
+## Exported fields
+
+The following fields are exported and can be referenced by other components:
+
+| Name       | Type              | Description                                               |
+| ---------- | ----------------- | --------------------------------------------------------- |
+| `receiver` | `MetricsReceiver` | A value that other components can use to send metrics to. |
+
+## Component health
+
+`prometheus.echo` is only reported as unhealthy if given an invalid configuration.
+In those cases, exported fields retain their last healthy values.
+
+## Debug information
+
+`prometheus.echo` doesn't expose any component-specific debug information.
+
+## Debug metrics
+
+`prometheus.echo` doesn't expose any component-specific debug metrics.
+
+## Example
+
+This example creates a metrics generation and inspection pipeline:
+
+```alloy
+prometheus.exporter.unix "default" {
+}
+
+prometheus.scrape "demo" {
+  targets    = prometheus.exporter.unix.default.targets
+  forward_to = [prometheus.echo.debug.receiver]
+}
+
+prometheus.echo "debug" {
+  format = "text"
+}
+```
+
+In this example:
+
+1. The `prometheus.exporter.unix` component exposes system metrics.
+1. The `prometheus.scrape` component scrapes those metrics.
+1. The `prometheus.echo` component receives the scraped metrics and writes them to stdout in Prometheus text format.
+
+When you run this configuration, you'll see the metrics being written to stdout, which is useful to:
+
+* Debug metric collection issues
+* Verify metric labels and values
+* Test metric transformations
+* Understand the structure of metrics in your pipeline
+
+### Example with OpenMetrics format
+
+```alloy
+prometheus.scrape "demo" {
+  targets = [
+    {"__address__" = "localhost:9090"},
+  ]
+  forward_to = [prometheus.echo.debug.receiver]
+}
+
+prometheus.echo "debug" {
+  format = "openmetrics"
+}
+```
+
+This example outputs metrics using the OpenMetrics format instead of the traditional Prometheus text format.
+
+<!-- START GENERATED COMPATIBLE COMPONENTS -->
+
+## Compatible components
+
+`prometheus.echo` has exports that can be consumed by the following components:
+
+- Components that consume [Prometheus `MetricsReceiver`](../../../compatibility/#prometheus-metricsreceiver-consumers)
+
+{{< admonition type="note" >}}
+Connecting some components may not be sensible or components may require further configuration to make the connection work correctly.
+Refer to the linked documentation for more details.
+{{< /admonition >}}
+
+<!-- END GENERATED COMPATIBLE COMPONENTS -->

internal/component/all/all.go

@@ -127,6 +127,7 @@ import (
 	_ "github.com/grafana/alloy/internal/component/otelcol/receiver/vcenter"                 // Import otelcol.receiver.vcenter
 	_ "github.com/grafana/alloy/internal/component/otelcol/receiver/zipkin"                  // Import otelcol.receiver.zipkin
 	_ "github.com/grafana/alloy/internal/component/otelcol/storage/file"                     // Import otelcol.storage.file
+	_ "github.com/grafana/alloy/internal/component/prometheus/echo"                          // Import prometheus.echo
 	_ "github.com/grafana/alloy/internal/component/prometheus/enrich"                        // Import prometheus.enrich
 	_ "github.com/grafana/alloy/internal/component/prometheus/exporter/apache"               // Import prometheus.exporter.apache
 	_ "github.com/grafana/alloy/internal/component/prometheus/exporter/azure"                // Import prometheus.exporter.azure