Add deprecation warning for InfluxDB

doc/administration/monitoring/performance/gitlab_configuration.md

 # GitLab Configuration
 
+CAUTION: **InfluxDB is deprecated in favor of Prometheus:**
+InfluxDB support is scheduled to be removed in GitLab 13.0.
+You are advised to use [Prometheus](../prometheus/index.md) instead.
+
 GitLab Performance Monitoring is disabled by default. To enable it and change any of its
 settings, navigate to the Admin area in **Settings > Metrics**
 (`/admin/application_settings`).

doc/administration/monitoring/performance/index.md

 # GitLab Performance Monitoring
 
+CAUTION: **InfluxDB is deprecated in favor of Prometheus:**
+InfluxDB support is scheduled to be removed in GitLab 13.0.
+You are advised to use [Prometheus](../prometheus/index.md) instead.
+
 GitLab comes with its own application performance measuring system as of GitLab
 8.4, simply called "GitLab Performance Monitoring". GitLab Performance Monitoring is available in both the
 Community and Enterprise editions.

doc/administration/monitoring/performance/influxdb_configuration.md

 # InfluxDB Configuration
 
+CAUTION: **InfluxDB is being deprecated in favor of Prometheus:**
+InfluxDB support is scheduled to be dropped in GitLab 13.0.
+You are advised to use [Prometheus](../prometheus/index.md) instead.
+
 The default settings provided by [InfluxDB] are not sufficient for a high traffic
 GitLab environment. The settings discussed in this document are based on the
 settings GitLab uses for GitLab.com, depending on your own needs you may need to

doc/administration/monitoring/performance/influxdb_schema.md

 # InfluxDB Schema
 
+CAUTION: **InfluxDB is deprecated in favor of Prometheus:**
+InfluxDB support is scheduled to be removed in GitLab 13.0.
+You are advised to use [Prometheus](../prometheus/index.md) instead.
+
 The following measurements are currently stored in InfluxDB:
 
 - `PROCESS_file_descriptors`