collab: Attach GitHub login to LLM spans (#16316)

This PR updates the LLM service to include the GitHub login on its
spans.

We need to pass this information through on the LLM token, so it will
temporarily be `None` until this change is deployed and new tokens have
been issued.

Release Notes:

- N/A

crates/collab/src/llm/token.rs

@@ -13,6 +13,12 @@ pub struct LlmTokenClaims {
     pub exp: u64,
     pub jti: String,
     pub user_id: u64,
+    // This field is temporarily optional so it can be added
+    // in a backwards-compatible way. We can make it required
+    // once all of the LLM tokens have cycled (~1 hour after
+    // this change has been deployed).
+    #[serde(default)]
+    pub github_user_login: Option<String>,
     pub is_staff: bool,
     pub plan: rpc::proto::Plan,
 }
@@ -22,6 +28,7 @@ const LLM_TOKEN_LIFETIME: Duration = Duration::from_secs(60 * 60);
 impl LlmTokenClaims {
     pub fn create(
         user_id: UserId,
+        github_user_login: String,
         is_staff: bool,
         plan: rpc::proto::Plan,
         config: &Config,
@@ -37,6 +44,7 @@ impl LlmTokenClaims {
             exp: (now + LLM_TOKEN_LIFETIME).timestamp() as u64,
             jti: uuid::Uuid::new_v4().to_string(),
             user_id: user_id.to_proto(),
+            github_user_login: Some(github_user_login),
             is_staff,
             plan,
         };