Expose Prometheus docs to the main README

[ci skip]

doc/README.md

@@ -50,7 +50,8 @@
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)
 - [Housekeeping](administration/housekeeping.md) Keep your Git repository tidy and fast.
-- [GitLab Performance Monitoring](administration/monitoring/performance/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics.
+- [GitLab performance monitoring with InfluxDB](administration/monitoring/performance/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics.
+- [GitLab performance monitoring with Prometheus](administration/monitoring/performance/prometheus.md) Configure GitLab and Prometheus for measuring performance metrics.
 - [Request Profiling](administration/monitoring/performance/request_profiling.md) Get a detailed profile on slow requests.
 - [Monitoring uptime](user/admin_area/monitoring/health_check.md) Check the server status using the health check endpoint.
 - [Debugging Tips](administration/troubleshooting/debug.md) Tips to debug problems when things go wrong

doc/administration/monitoring/performance/introduction.md

@@ -12,6 +12,11 @@ documents in order to understand and properly configure GitLab Performance Monit
 - [InfluxDB Schema](influxdb_schema.md)
 - [Grafana Install/Configuration](grafana_configuration.md)
 
+>**Note:**
+Omnibus GitLab 8.16 includes Prometheus as an additional tool to collect
+metrics. It will eventually replace InfluxDB when their metrics collection is
+on par. Read more in the [Prometheus documentation](prometheus.md).
+
 ## Introduction to GitLab Performance Monitoring
 
 GitLab Performance Monitoring makes it possible to measure a wide variety of statistics