How do I enable the ETag header? I'd like my JSON:API responses to return the e-tag header so that consuming remote sites can determine (e.g. with a HEAD request) if they need to ditch their cached responses and re-fetch.  There is no last-modified or e-tag headers being sent at all - for any Drupal page for that matter.  How can I enable this?  Thanks!

I do see X-Drupal-Dynamic-Cache and X-Drupal-Cache headers, but this isn't enough to go on.  

Running Drupal 8.6.10 and JSON:API 8.x-2.4.
I changed the "Browser and proxy cache maximum age" setting in Configuration > Development > Performance and it appeared to do the trick.  It was previously set to "no caching".