prometheus: Implement basic Prometheus support

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

docs/prometheus.md

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+# mgmt
+
+<!--
+Mgmt
+Copyright (C) 2013-2016+ James Shubin and the project contributors
+Written by James Shubin <james@shubin.ca> and the project contributors
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+-->
+
+## Prometheus support
+
+Mgmt comes with a built-in prometheus support. It is disabled by default, and
+can be enabled with the `--prometheus` command line switch.
+
+By default, the prometheus instance will listen on [`127.0.0.1:9233`][pd]. You can
+change this setting by using the `--prometheus-listen` cli option:
+
+To have mgmt prometheus bind interface on 0.0.0.0:45001, use:
+`./mgmt r --prometheus --prometheus-listen :45001`
+
+## Metrics
+
+Mgmt exposes two kinds of resources: _go_ metrics and _mgmt_ metrics.
+
+### go metrics
+
+We use the [prometheus go_collector][pgc] to expose go metrics. Those metrics
+are mainly useful for debugging and perf testing.
+
+### etcd metrics
+
+mgmt exposes etcd metrics. Read more in the [upstream documentation][etcdm]
+
+### mgmt metrics
+
+Here is a list of the metrics we provide:
+
+- `mgmt_resources_total`: The number of resources that mgmt is managing
+- `mgmt_checkapply_total`: The number of checkapplies that mgmt has run.
+- `mgmt_failures_total`: The number of resources that have failed
+- `mgmt_failures_current`: The number of resources that have failed
+
+For each metric, you will get some extra labels:
+
+- `resource_type`: The type of mgmt resource
+
+## Alerting
+
+You can use prometheus to alert you upon changes or failures. We do not provide
+such templates yet, but we plan to provide some examples in this repository.
+Patches welcome!
+
+## Grafana
+
+We do not have grafana dashboards yet. Patches welcome!
+
+## External resources
+
+- [prometheus website](https://prometheus.io/)
+- [prometheus documentation](https://prometheus.io/docs/introduction/overview/)
+- [prometheus best practices regarding metrics
+  naming](https://prometheus.io/docs/practices/naming/)
+- [grafana website](http://grafana.org/)
+
+[pgc]: https://github.com/prometheus/client_golang/blob/master/prometheus/go_collector.go
+[etcdm]: https://coreos.com/etcd/docs/latest/metrics.html
+[pd]: https://github.com/prometheus/prometheus/wiki/Default-port-allocation