Merge pull request #48 from overleaf/csh-add-README

Add basic README

libraries/metrics/README.md

@@ -0,0 +1,30 @@
+overleaf/metrics-module
+=======================
+
+Wrappers the [prom-client](https://github.com/siimon/prom-client) npm module to provide [Prometheus](https://prometheus.io/) metrics at `/metrics`.
+
+Use:
+```
+const metrics = require('@overleaf/metrics')
+metrics.initialize('myapp')
+
+const express = require('express')
+const app = express()
+metrics.injectMetricsRoute(app)
+```
+Request logging can be enabled:
+```
+const logger = require('logger-sharelatex')
+...
+app.use(metrics.http.monitor(logger))
+```
+
+The metrics module can be configured through the following environment variables:
+
+* `DEBUG_METRICS` - enables display of debugging messages to the console.
+* `ENABLE_TRACE_AGENT` - enables @google-cloud/trace-agent on Google Cloud
+* `ENABLE_DEBUG_AGENT` - enables @google-cloud/debug-agent on Google Cloud
+* `ENABLE_PROFILE_AGENT` - enables @google-cloud/profiler on Google Cloud
+* `METRICS_COMPRESSION_LEVEL` - sets the [compression level](https://www.npmjs.com/package/compression#level) for `/metrics`
+* `STACKDRIVER_LOGGING` - toggles the request logging format
+* `UV_THREADPOOL_SIZE` - sets the libuv [thread pool](http://docs.libuv.org/en/v1.x/threadpool.html) size