Optimize link selection metadata

This reduces the size of metadata fields used in link selection as they
can substantially increase the size of HELLO messages.

The abbreviations "rel" and "prio" are used for "priorities" and
"reliability" respectively. Furthermore, the numbers 0 and 1 are used to
denote "best_effort" and "reliable" reliabilities.

commons/zenoh-protocol/src/core/endpoint.rs

@@ -187,8 +187,8 @@ impl fmt::Debug for AddressMut<'_> {
 pub struct Metadata<'a>(pub(super) &'a str);
 
 impl<'a> Metadata<'a> {
-    pub const RELIABILITY: &'static str = "reliability";
-    pub const PRIORITIES: &'static str = "priorities";
+    pub const RELIABILITY: &'static str = "rel";
+    pub const PRIORITIES: &'static str = "prio";
 
     pub fn as_str(&self) -> &'a str {
         self.0

commons/zenoh-protocol/src/core/mod.rs

@@ -457,24 +457,14 @@ impl TryFrom<u8> for Priority {
 #[derive(Debug, Default, Copy, Clone, PartialEq, Eq, Hash, Serialize)]
 #[repr(u8)]
 pub enum Reliability {
+    BestEffort = 0,
     #[default]
-    Reliable,
-    BestEffort,
+    Reliable = 1,
 }
 
 impl Reliability {
     pub const DEFAULT: Self = Self::Reliable;
 
-    const BEST_EFFORT_STR: &'static str = "best_effort";
-    const RELIABLE_STR: &'static str = "reliable";
-
-    pub fn as_str(&self) -> &str {
-        match self {
-            Reliability::BestEffort => Reliability::BEST_EFFORT_STR,
-            Reliability::Reliable => Reliability::RELIABLE_STR,
-        }
-    }
-
     #[cfg(feature = "test")]
     pub fn rand() -> Self {
         use rand::Rng;
@@ -508,6 +498,12 @@ impl From<Reliability> for bool {
     }
 }
 
+impl Display for Reliability {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", *self as u8)
+    }
+}
+
 #[derive(Debug)]
 pub struct InvalidReliability {
     found: String,
@@ -518,8 +514,8 @@ impl Display for InvalidReliability {
         write!(
             f,
             "invalid Reliability string, expected `{}` or `{}` but found {}",
-            Reliability::BEST_EFFORT_STR,
-            Reliability::RELIABLE_STR,
+            Reliability::Reliable as u8,
+            Reliability::BestEffort as u8,
             self.found
         )
     }
@@ -532,12 +528,20 @@ impl FromStr for Reliability {
     type Err = InvalidReliability;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        match s {
-            Reliability::RELIABLE_STR => Ok(Reliability::Reliable),
-            Reliability::BEST_EFFORT_STR => Ok(Reliability::BestEffort),
-            other => Err(InvalidReliability {
-                found: other.to_string(),
-            }),
+        let Ok(desc) = s.parse::<u8>() else {
+            return Err(InvalidReliability {
+                found: s.to_string(),
+            });
+        };
+
+        if desc == Reliability::BestEffort as u8 {
+            Ok(Reliability::BestEffort)
+        } else if desc == Reliability::Reliable as u8 {
+            Ok(Reliability::Reliable)
+        } else {
+            return Err(InvalidReliability {
+                found: s.to_string(),
+            });
         }
     }
 }

io/zenoh-link-commons/src/lib.rs

@@ -117,7 +117,7 @@ impl Link {
         let mut locator = locator.clone();
         let mut metadata = locator.metadata_mut();
         reliability
-            .map(|r| metadata.insert(Metadata::RELIABILITY, r.as_str()))
+            .map(|r| metadata.insert(Metadata::RELIABILITY, r.to_string()))
             .transpose()
             .expect("adding `reliability` to Locator metadata should not fail");
         priorities