Follow conversation by push notifications case to monitor/improvement

This is related to an observed behaviour while testing this PR here

Slack internal ref: p1633941032482100-slack-C01CW1VMLAF

If the site that you want to follow a conversation on, has this below site global setting in the reader active

then the /sites/$site_id/posts/$post_id/subscribers/mine/update endpoint cannot set to true the receive_notifications field; thus in this condition currently an error is got back and the push notifications cannot be enabled.

This would require some changes in the endpoint but numbers for this scenario looking at the calypso_reader_comment_emails_toggle_on seems pretty low (if we are not wrong, like only 0.078% of MAUs are using that feature). We have setup a change in the endpoint (diff D68317) to bump a counter that we can review to define eventual action on the endpoint.

Props to @aerych for the time and advices looking into this 🙇 !

Start with the referenced PR and the /sites/$site_id/posts/$post_id/subscribers/mine/update endpoint behavior; the issue does not name a repository file or test. Confirm the receive_notifications=true failure under the reader setting, then verify that the monitoring counter captures this case so a decision on the endpoint can be made.