ISW - Add tracking for selected routing option

[grotlue]

Description

closes https://github.com/Graylog2/graylog-plugin-enterprise/issues/9390
/nocl

Add tracking to answer the following product questions defined in https://github.com/Graylog2/graylog-plugin-enterprise/issues/9390

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.