feat(eap): emit item type counts (#7548)

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):
#7479 (merged)
- start keeping track of how many items by type are being processed in
the EAPItems processor: #7481
- (this change) emit the actual metric at the end of a batch write

Author: onewland

rust_snuba/src/strategies/clickhouse/writer_v2.rs

@@ -62,6 +62,7 @@ fn clickhouse_task_runner(
             counter!("insertions.batch_write_bytes", num_bytes as i64);
             counter!("insertions.batch_write_msgs", batch_len as i64);
             empty_batch.record_message_latency();
+            empty_batch.emit_item_type_metrics();
 
             Ok(empty_message.replace(empty_batch))
         })

rust_snuba/src/types.rs

@@ -360,6 +360,18 @@ impl<R> BytesInsertBatch<R> {
         self.sentry_received_timestamp
             .send_metric(write_time, "sentry_received_latency");
     }
+
+    pub fn emit_item_type_metrics(&self) {
+        use sentry_arroyo::counter;
+
+        for (item_type, count) in &self.item_type_metrics.counts {
+            counter!(
+                "insertions.item_type_count",
+                *count as i64,
+                "item_type" => item_type.to_string()
+            );
+        }
+    }
 }
 
 impl BytesInsertBatch<RowData> {