Instrument viewing merge request diffs file by file

What does this MR do?

In order to understand how are users are using the view file by file feature, we need to track:

• How many users viewed MRs in file by file mode (unique by user id).
• How many MRs were viewed in file by file mode (unique by MR id).
• How many MRs were viewed (in any mode; unique by MR id).

This is tracked on each request to diffs_batch.json since that's when we show the file by file controls.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #247184 (closed)