Use header-only usage server timing

crates/aether-contracts/src/lib.rs

@@ -16,4 +16,6 @@ pub use plan::{
     TRANSPORT_HTTP_MODE_HTTP1_ONLY, TRANSPORT_POOL_SCOPE_KEY,
 };
 pub use result::{ExecutionResult, ExecutionTelemetry, ResponseBody};
-pub use usage::{ExecutionStreamTerminalSummary, StandardizedUsage};
+pub use usage::{
+    ExecutionStreamTerminalSummary, StandardizedUsage, USAGE_SERVER_NOW_UNIX_MS_HEADER,
+};

crates/aether-contracts/src/usage.rs

@@ -2,6 +2,8 @@ use std::collections::BTreeMap;
 
 use serde::{Deserialize, Serialize};
 
+pub const USAGE_SERVER_NOW_UNIX_MS_HEADER: &str = "x-aether-server-now-unix-ms";
+
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default)]
 pub struct StandardizedUsage {
     pub input_tokens: i64,