Add ability to rate limit the number of attachments

[jjbayer]

Attachment quotas and rate limits are currently defined in bytes, so we have no way to prevent an abusively high number of very small attachments.

Instead of introducing yet another data category for attachment items, we should attempt to rethink quota definitions:

We have a couple of cases now where we'd actually like to track several stats per data category. So the category is "attachment", but the stats we track are: count, cumulative size. Or category is "span" but the stats we track are: received count, stored count. Category is "bucket", stats are: count, cardinality (that's an odd one).

Relay

Beta Give feedback

1. Extend the Quota type with a unit field
2. Count both attachment bytes and number of attachment items in the EnvelopeSummary
3. Enforce rate limits on both units

Sentry

Beta Give feedback

1. Define an abuse limit for attachment items

ref: INC-928 > IA-61