debug.zip: crdb_internal.index_usage_statistics.txt only has data for system database

**Is your feature request related to a problem? Please describe.**
The index usage statistics in a debug.zip (in the file crdb_internal.index_usage_statistics.txt) only contains information about indexes in the system database.

**Describe the solution you'd like**
crdb_internal.index_usage_statistics.txt should contain information about indexes in all databases.

**Describe alternatives you've considered**
The only alternative is to extract the index usage information from the running cluster.

**Additional context**
crdb_internal.create_statements.txt contains create statements for all databases (in a running cluster only the create statements or the current database are visible from crdb_internal.create_statements) so maybe the same mechanism can be used to gather index usage information into the debug.zip.

Jira issue: CRDB-23097