Clarify usage for event tracking and instrumentation

doc/development/event_tracking/index.md

-# Event tracking
+# Product Analytics
 
 At GitLab, we encourage event tracking so we can iterate on and improve the project and user experience.
 
@@ -44,6 +44,8 @@ From the backend, the events that are tracked will likely consist of things like
 
 See [Backend tracking guide](backend.md).
 
+Also, see [Application performance metrics](../instrumentation.md) if you are after instrumenting application performance metrics.
+
 ## Enabling tracking
 
 Tracking can be enabled at:

doc/development/instrumentation.md

-# Instrumenting Ruby Code
+# Application Performance Metrics for Ruby Code
 
 [GitLab Performance Monitoring](../administration/monitoring/performance/index.md) allows instrumenting of both methods and custom
 blocks of Ruby code. Method instrumentation is the primary form of
 instrumentation with block-based instrumentation only being used when we want to
 drill down to specific regions of code within a method.
 
+Please refer to [Product analytics](event_tracking/index.md) if you are after tracking product usage patterns.
+
 ## Instrumenting Methods
 
 Instrumenting methods is done by using the `Gitlab::Metrics::Instrumentation`