feat: add metric to track the number of turns with memory usage (#18662)

Add a metric `codex.turn.memory` to know if a turn used memories or not.
This is not part of the other turn metrics as a label to limit
cardinality

codex-rs/core/src/state/turn.rs

@@ -105,6 +105,7 @@ pub(crate) struct TurnState {
     mailbox_delivery_phase: MailboxDeliveryPhase,
     granted_permissions: Option<PermissionProfile>,
     pub(crate) tool_calls: u64,
+    pub(crate) has_memory_citation: bool,
     pub(crate) token_usage_at_turn_start: TokenUsage,
 }