add get_history function to Diagnostic (#2772)

# Objective

- Allow access to diagnostic history value.
- Fixes #2771.

## Solution

- Add Diagnostic::get_history function.

crates/bevy_diagnostic/src/diagnostic.rs

@@ -119,6 +119,14 @@ impl Diagnostic {
     pub fn get_max_history_length(&self) -> usize {
         self.max_history_length
     }
+
+    pub fn values(&self) -> impl Iterator<Item = &f64> {
+        self.history.iter().map(|x| &x.value)
+    }
+
+    pub fn measurements(&self) -> impl Iterator<Item = &DiagnosticMeasurement> {
+        self.history.iter()
+    }
 }
 
 /// A collection of [Diagnostic]s