feat(eap): emit item type counts

[onewland]

Emit how many events by type are being processed by the EAP consumer. This is being split into 3 changes:

• change how we build BytesInsertBatch to support a builder pattern (so it's more simple to omit dataset-specific fields): https://github.com/getsentry/snuba/pull/7479 (merged)
• start keeping track of how many items by type are being processed in the EAPItems processor: https://github.com/getsentry/snuba/pull/7481
• (this change) emit the actual metric at the end of a batch write