Add global metrics registry, pass to BeaconState

beacon_node/http_server/src/key.rs

@@ -1,5 +1,6 @@
 use beacon_chain::{BeaconChain, BeaconChainTypes};
 use iron::typemap::Key;
+use prometheus::Registry;
 use std::marker::PhantomData;
 use std::sync::Arc;
 
@@ -10,3 +11,9 @@ pub struct BeaconChainKey<T> {
 impl<T: BeaconChainTypes + 'static> Key for BeaconChainKey<T> {
     type Value = Arc<BeaconChain<T>>;
 }
+
+pub struct MetricsRegistryKey;
+
+impl Key for MetricsRegistryKey {
+    type Value = Registry;
+}