Update doc comments (#2149)

Author: utpilla

opentelemetry/src/metrics/instruments/mod.rs

@@ -78,6 +78,10 @@ impl<'a, T> HistogramBuilder<'a, T> {
     }
 
     /// Set the boundaries for this histogram.
+    ///
+    /// Setting boundaries is optional. By default, the boundaries are set to:
+    ///
+    /// `[0.0, 5.0, 10.0, 25.0, 50.0, 75.0, 100.0, 250.0, 500.0, 750.0, 1000.0, 2500.0, 5000.0, 7500.0, 10000.0]`
     pub fn with_boundaries(mut self, boundaries: Vec<f64>) -> Self {
         self.boundaries = Some(boundaries);
         self