Using Cloud CDN in front of a Compute Engine instance group for static HTTPS content, how can you improve cache hit ratio?

Customizing cache keys to omit the protocol (HTTP vs. HTTPS) improves cache hit ratio. By default, Cloud CDN caches separate objects for HTTP and HTTPS requests, even if the content is identical. Omitting the protocol means both HTTP and HTTPS requests for the same URL path will hit the same cached object, increasing efficiency. Shortening expiration times reduces cache hit ratio by forcing more frequent revalidation. 'Cache-Region' is not a standard Cloud CDN header for cache key manipulation. While moving static content to Cloud Storage is a good practice for scalability and cost, it doesn't directly improve cache hit ratio for an existing Compute Engine setup; the question asks how to improve the ratio for the current setup.