Refactor HTTP metrics. (#1249)

This PR refactors how response for the HTTP metrics endpoint is being
assembled. It uses a very thin layer on top of the bare Prometheus
protocol instead of manually writing all the content.

Ideally, this doesn’t change any of the metrics. It does, however,
slightly change the formatting.

src/commons/api/ca.rs

@@ -1,11 +1,11 @@
 //! Common data types for Certificate Authorities, defined here so that the
 //! CLI can have access without needing to depend on the full krill_ca module.
 
+use std::{fmt, ops, str};
+use std::collections::hash_map;
 use std::collections::HashMap;
-use std::ops::{self};
 use std::str::FromStr;
 use std::sync::Arc;
-use std::{fmt, str};
 
 use base64::engine::general_purpose::STANDARD as BASE64_ENGINE;
 use base64::engine::Engine as _;
@@ -962,9 +962,7 @@ impl ParentStatuses {
         self.0.get(parent)
     }
 
-    pub fn iter(
-        &self,
-    ) -> impl Iterator<Item = (&ParentHandle, &ParentStatus)> {
+    pub fn iter(&self) -> hash_map::Iter<ParentHandle, ParentStatus> {
         self.0.iter()
     }
 
@@ -1062,14 +1060,22 @@ impl ParentStatuses {
 
 impl IntoIterator for ParentStatuses {
     type Item = (ParentHandle, ParentStatus);
-    type IntoIter =
-        std::collections::hash_map::IntoIter<ParentHandle, ParentStatus>;
+    type IntoIter = hash_map::IntoIter<ParentHandle, ParentStatus>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.0.into_iter()
     }
 }
 
+impl<'a> IntoIterator for &'a ParentStatuses {
+    type Item = (&'a ParentHandle, &'a ParentStatus);
+    type IntoIter = hash_map::Iter<'a, ParentHandle, ParentStatus>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}
+
 impl fmt::Display for ParentStatuses {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         for (parent, status) in self.0.iter() {