[ios] Add metric to count form submissions upstream, before processing

The idea is to count the number of form submission events that occurred
before doing any processing. We'd like knowing where we're losing form
submissions when moving autofill to the isolated world.

The solution is honestly a bit too complex as using the quotas probably
covers 95% of form submission events but I'd like testing an approach
that allows reporting and throttling events without losses. This will be
a POC basically

We may want to apply that approach for throttling frequent events like
typing (i.e. the 'input' and 'change', see form_handlers.ts) for
instance.

If we find that
Autofill.iOS.FormActivity.SubmissionDetectedBeforeProcessing.BatchSize
is close to 1 most of the time, we will remove batching and stick with
quotas.

Fixed: 426661129
Change-Id: I3ca0e7429093ac3975e9b2e5c7997f50b82414d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6652905
Reviewed-by: Noémie St-Onge <noemies@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Cr-Commit-Position: refs/heads/main@{#1478732}