refactor: tidy consumption metrics (#4860)

Tidying up I've been wanting to do for some time.

Follow-up to #4857.

libs/consumption_metrics/src/lib.rs

@@ -57,7 +57,7 @@ pub struct Event<Extra> {
     pub extra: Extra,
 }
 
-pub fn idempotency_key(node_id: String) -> String {
+pub fn idempotency_key(node_id: &str) -> String {
     format!(
         "{}-{}-{:04}",
         Utc::now(),
@@ -71,6 +71,6 @@ pub const CHUNK_SIZE: usize = 1000;
 // Just a wrapper around a slice of events
 // to serialize it as `{"events" : [ ] }
 #[derive(serde::Serialize)]
-pub struct EventChunk<'a, T> {
-    pub events: &'a [T],
+pub struct EventChunk<'a, T: Clone> {
+    pub events: std::borrow::Cow<'a, [T]>,
 }