message: Expose max_msgs_per_subject StreamConfig attribute
diff: @@ -150,6 +150,8 @@ pub struct StreamConfig {
  pub max_bytes: i64,
  /// How large the Stream may become in total messages before the configured discard policy kicks in
  pub max_msgs: i64,
  + /// Maximum amount of messages to keep per subject
  + pub max_msgs_per_subject: i64,
  /// When a Stream has reached its configured `max_bytes` or `max_msgs`, this policy kicks in.
  /// `DiscardPolicy::New` refuses new messages or `DiscardPolicy::Old` (default) deletes old messages to make space
  pub discard: DiscardPolicy,
  