Monitoring documentation (#1402)

Add docs folder.
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Add monitoring documentation.

docs/Monitoring.md

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+# Monitoring Eclair
+
+Eclair uses [Kamon](https://kamon.io/) to generate metrics and spans. Kamon automatically collects
+many akka-related metrics and performance counters from the underlying system.
+
+Kamon uses an agent ([Kanela](https://github.com/kamon-io/kanela)) that runs alongside eclair and
+periodically sends metrics and spans to a monitoring backend.
+
+## Enabling monitoring with Kamon APM
+
+Monitoring is disabled by default. To enable monitoring with Kamon's hosted platform, create an
+account on their website and add the following to your `eclair.conf`:
+
+```config
+eclair.enable-kamon=true
+
+kamon {
+
+  apm {
+    # Put the API key obtained from your Kamon account.
+    api-key=XXXXXXX
+  }
+
+  trace {
+    # Configures a sampler that decides which traces should be reported to the trace backends. The possible values are:
+    #   - always: report all traces (will impact application performance).
+    #   - never:  don't report any trace.
+    #   - random: randomly decide using the probability defined in the random-sampler.probability setting.
+    #   - adaptive: keeps dynamic samplers for each operation while trying to achieve a set throughput goal.
+    sampler = "random"
+  }
+
+}
+```
+
+When starting eclair, you should enable the Kanela agent:
+
+```sh
+eclair.sh -with-kanela
+```
+
+And 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
+
+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,
+don't hesitate to ask around or send a PR.
+
+## Example metrics
+
+Apart from akka and system metrics, eclair generates a lot of lightning metrics. The following
+metrics are just a small sample of all the metrics we provide:
+
+* Number of local channels, grouped by their current state (offline, normal, closing, etc)
+* Relayed payments and fees collected
+* Number of connected peers
+* Bitcoin wallet balance
+* Various metrics about the public graph (nodes, channels, updates, etc)