feat(router_env): making metric flow as a trait for extensibility (#797)

crates/router_env/src/logger/types.rs

@@ -164,6 +164,12 @@ pub enum Flow {
     ApiKeyList,
 }
 
+///
+/// Trait for providing generic behaviour to flow metric
+///
+pub trait FlowMetric: ToString + std::fmt::Debug {}
+impl FlowMetric for Flow {}
+
 /// Category of log event.
 #[derive(Debug)]
 pub enum Category {