Add docs and issue template for performance indicator changes

.gitlab/issue_templates/Performance Indicator Metric.md

+<!--
+Performance Indicator Metric issues are used for adding, updating, or removing performance indicator type in Service Ping metrics.
+
+Please title your issue with the following format: "{action}(Add|Update|Remove) Metric name as performance indicator"
+
+Example of title: "Add static_site_editor_views as gmau"
+
+-->
+
+## Summary
+
+<!--
+Summary of the changes
+-->
+
+## Tasks
+
+[ ] [Link to metric definition]()
+[ ] Create issue in GitLab Data Team project using [Product Performance Indicator template](https://gitlab.com/gitlab-data/analytics/-/issues/new?issuable_template=Product%20Performance%20Indicator%20Template)
+
+/label ~"product intelligence" "~Data Warehouse::Impact Check"

doc/development/service_ping/performance_indicator_metrics.md

+---
+stage: Growth
+group: Product Intelligence
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Performance Indicator Metrics guide
+
+This guide describes how to use metrics definitions to define [performance indicator](https://about.gitlab.com/handbook/product/product-intelligence-guide/#implementing-product-performance-indicators) metrics.
+
+To use a metric definition to manage a performance indicator:
+
+1. Create a new issue and use the [Performance Indicator Metric issue template](https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Performance%20Indicator%20Metric.md).
+1. Use labels `~"product intelligence"`, `"~Data Warehouse::Impact Check"`.
+1. Create a merge request that includes changes related only to the metric performance indicator.
+1. Update the metric definition `performance_indicator_type` [field](metrics_dictionary.md#metrics-definition-and-validation).
+1. Create an issue in GitLab Data Team project with the [Product Performance Indicator template](https://gitlab.com/gitlab-data/analytics/-/issues/new?issuable_template=Product%20Performance%20Indicator%20Template).