Make sure we include end_value in generated Prometheus buckets (#3363)

Right now we're generating the buckets, but it's not in an inclusive way, and it should be. For latency metrics, for example, you have `end_value` being 500ms, but one of the buckets is 159. Since we're using a multiplication factor of 3, the next bucket would go over 500ms, so we don't include it...

Make sure we always include the `end_value` bucket

CI + start a network locally and see the correct buckets exported

- Nothing to do / These changes follow the usual release cycle.

.cursor/rules/location.mdc

@@ -0,0 +1,75 @@
+---
+description: Cursor Rules Location
+globs: *.mdc
+---
+# Cursor Rules Location
+
+Rules for placing and organizing Cursor rule files in the repository.
+
+<rule>
+name: cursor_rules_location
+description: Standards for placing Cursor rule files in the correct directory
+filters:
+  # Match any .mdc files
+  - type: file_extension
+    pattern: "\\.mdc$"
+  # Match files that look like Cursor rules
+  - type: content
+    pattern: "(?s)<rule>.*?</rule>"
+  # Match file creation events
+  - type: event
+    pattern: "file_create"
+
+actions:
+  - type: reject
+    conditions:
+      - pattern: "^(?!\\.\\/\\.cursor\\/rules\\/.*\\.mdc$)"
+        message: "Cursor rule files (.mdc) must be placed in the .cursor/rules directory"
+
+  - type: suggest
+    message: |
+      When creating Cursor rules:
+
+      1. Always place rule files in PROJECT_ROOT/.cursor/rules/:
+         ```
+         .cursor/rules/
+         ├── your-rule-name.mdc
+         ├── another-rule.mdc
+         └── ...
+         ```
+
+      2. Follow the naming convention:
+         - Use kebab-case for filenames
+         - Always use .mdc extension
+         - Make names descriptive of the rule's purpose
+
+      3. Directory structure:
+         ```
+         PROJECT_ROOT/
+         ├── .cursor/
+         │   └── rules/
+         │       ├── your-rule-name.mdc
+         │       └── ...
+         └── ...
+         ```
+
+      4. Never place rule files:
+         - In the project root
+         - In subdirectories outside .cursor/rules
+         - In any other location
+
+examples:
+  - input: |
+      # Bad: Rule file in wrong location
+      rules/my-rule.mdc
+      my-rule.mdc
+      .rules/my-rule.mdc
+
+      # Good: Rule file in correct location
+      .cursor/rules/my-rule.mdc
+    output: "Correctly placed Cursor rule file"
+
+metadata:
+  priority: high
+  version: 1.0
+</rule>

linera-base/src/prometheus_util.rs

@@ -4,8 +4,8 @@
 //! This module defines util functions for interacting with Prometheus (logging metrics, etc)
 
 use prometheus::{
-    histogram_opts, register_histogram_vec, register_int_counter_vec, Error, HistogramVec,
-    IntCounterVec, Opts,
+    exponential_buckets, histogram_opts, register_histogram_vec, register_int_counter_vec, Error,
+    HistogramVec, IntCounterVec, Opts,
 };
 
 use crate::time::Instant;
@@ -38,6 +38,27 @@ pub fn register_histogram_vec(
     register_histogram_vec!(histogram_opts, label_names)
 }
 
+/// Construct the bucket interval starting from a value and an ending value.
+pub fn bucket_interval(start_value: f64, end_value: f64) -> Option<Vec<f64>> {
+    let quot = end_value / start_value;
+    let factor = 3.0_f64;
+    let count_approx = quot.ln() / factor.ln();
+    let count = count_approx.round() as usize;
+    let mut buckets = exponential_buckets(start_value, factor, count)
+        .expect("Exponential buckets creation should not fail!");
+    if let Some(last) = buckets.last() {
+        if *last < end_value {
+            buckets.push(end_value);
+        }
+    }
+    Some(buckets)
+}
+
+/// Construct the latencies starting from 0.0001 and ending at the maximum latency
+pub fn bucket_latencies(max_latency: f64) -> Option<Vec<f64>> {
+    bucket_interval(0.0001_f64, max_latency)
+}
+
 /// A guard for an active latency measurement.
 ///
 /// Finishes the measurement when dropped, and then updates the `Metric`.