Add metrics for 3pid validation attempts vs successes.

This issue has been migrated from [#8709](https://github.com/matrix-org/synapse/issues/8709).

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This is primarily to help see if emails sent are having delivery issues (i.e. are they getting spam canned).

Some potential metrics:

1. Number of emails sent for registration, password reset, etc
1. Number of times clients have asked for the emails to be resent (and how many times they were requested to be resent, as a histogram maybe?)
1. Number of these sessions that were validated, i.e. number of emails that the user clicked the link in