Expose scraping endpoint for prometheus metrics (#2321)

Allow eclair to expose a metrics scraping endpoint for Prometheus
for users who don't want to rely on Kamon's hosted infrastructure. 

Co-authored-by: Bastien Teinturier <31281497+t-bast@users.noreply.github.com>

docs/Monitoring.md

@@ -33,6 +33,7 @@ kamon {
 }
 ```
 
+
 When starting eclair, you should enable the Kanela agent:
 
 ```sh
@@ -42,13 +43,48 @@ eclair.sh -with-kanela
 Your eclair node will start exporting monitoring data to Kamon.
 You can then start creating dashboards, graphs and alerts directly on Kamon's website.
 
-## Enabling monitoring with a different backend
+## Enabling monitoring with Prometheus
 
 Kamon supports many other monitoring [backends](https://kamon.io/docs/latest/reporters/).
 This can be useful for nodes that don't want to export any data to third-party servers.
 
-No specific work has been done yet in eclair to support these backends. If you'd like to use them,
+Eclair currently supports exporting metrics to [Prometheus](https://kamon.io/docs/latest/reporters/prometheus/).
-don't hesitate to ask around or send a PR.
+To enable monitoring with Prometheus, add the following to your `eclair.conf`:
+
+```config
+eclair.enable-kamon=true
+
+// The Kamon APM reporter is enabled by default, but it won't work with Prometheus, so we disable it.
+kamon.modules {
+  apm-reporter {
+    enabled = false
+  }
+}
+
+kamon {
+  prometheus {
+    start-embedded-http-server = yes
+    embedded-server {
+      hostname = 0.0.0.0
+      port = <port to expose to prometheus>
+    }
+  }
+}
+```
+
+You should then configure your Prometheus process to scrape metrics from the exposed http server. 
+* Download Prometheus [here](https://prometheus.io/download/).
+* Add the following configuration to the `prometheus.yml` file (see the [Prometheus documentation](https://prometheus.io/docs/prometheus/latest/configuration/configuration/) for more details)
+
+```prometheus.yml
+global:
+  scrape_interval:     15s # By default, scrape targets every 15 seconds.
+
+scrape_configs:
+  - job_name: 'eclair'
+    static_configs:
+      - targets: ['<url of the eclair http embedded server>']
+ ```
 
 ## Example metrics
 
@@ -60,3 +96,4 @@ metrics are just a small sample of all the metrics we provide:
 * Number of connected peers
 * Bitcoin wallet balance
 * Various metrics about the public graph (nodes, channels, updates, etc)
+

eclair-core/pom.xml

@@ -270,6 +270,11 @@
             <version>1.5.0</version>
         </dependency>
         <!-- MONITORING -->
+        <dependency>
+            <groupId>io.kamon</groupId>
+            <artifactId>kamon-prometheus_${scala.version.short}</artifactId>
+            <version>2.5.4</version>
+        </dependency>
         <dependency>
             <groupId>io.kamon</groupId>
             <artifactId>kamon-core_${scala.version.short}</artifactId>

eclair-node/src/main/resources/application.conf

@@ -43,6 +43,15 @@ kamon.instrumentation.akka {
     }
   }
 }
+// See documentation here: https://kamon.io/docs/latest/reporters/prometheus/
+kamon.prometheus {
+  // If you want to expose a scraping endpoint for Prometheus metrics, set this to true.
+  start-embedded-http-server = no
+  embedded-server {
+    hostname = 0.0.0.0
+    port = 9095
+  }
+}
 
 kanela.modules {
   jdbc {