metrics: return histograms for time metrics

create local type MetricsMap
implement receive_metric function for local types
add error management
iterate logarithmically (base 2) to produce prometheus buckets

command/src/command.proto

@@ -602,13 +602,15 @@ message BackendMetrics {
     map<string, FilteredMetrics> metrics = 2;
 }
 
+// A metric, in a "filtered" format, which means: sendable to outside programs.
 message FilteredMetrics {
     oneof inner {
         uint64 gauge = 1;
         int64 count = 2;
         uint64 time = 3;
         Percentiles percentiles = 4;
         FilteredTimeSerie time_serie = 5;
+        FilteredHistogram histogram = 6;
     }
 }
 
@@ -618,7 +620,6 @@ message FilteredTimeSerie {
     repeated uint32 last_hour = 3;
 }
 
-
 message Percentiles {
     required uint64 samples = 1;
     required uint64 p_50 = 2;
@@ -631,6 +632,20 @@ message Percentiles {
     required uint64 sum = 9;
 }
 
+// a histogram meant to be translated to prometheus
+message FilteredHistogram {
+    required uint64 sum = 1;
+    required uint64 count = 2;
+    repeated Bucket buckets = 3;
+}
+
+// a prometheus histogram bucket
+message Bucket {
+    required uint64 count = 1;
+    // upper range of the bucket (le = less or equal)
+    required uint64 le = 2;
+}
+
 message RequestCounts {
     map<string, int32> map = 1;
 }

command/src/proto/display.rs

@@ -25,6 +25,8 @@ use crate::{
     AsString,
 };
 
+use super::command::FilteredHistogram;
+
 impl Display for CertificateAndKey {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         let versions = self.versions.iter().fold(String::new(), |acc, tls_v| {
@@ -252,6 +254,7 @@ fn print_proxy_metrics(proxy_metrics: &BTreeMap<String, FilteredMetrics>) {
     let filtered = filter_metrics(proxy_metrics);
     print_gauges_and_counts(&filtered);
     print_percentiles(&filtered);
+    print_histograms(&filtered);
 }
 
 fn print_worker_metrics(worker_metrics: &WorkerMetrics) -> Result<(), DisplayError> {
@@ -392,6 +395,56 @@ fn print_percentiles(filtered_metrics: &BTreeMap<String, FilteredMetrics>) {
     percentile_table.printstd();
 }
 
+fn print_histograms(filtered_metrics: &BTreeMap<String, FilteredMetrics>) {
+    let histograms: BTreeMap<String, FilteredHistogram> = filtered_metrics
+        .iter()
+        .filter_map(|(name, metric)| match metric.inner.clone() {
+            Some(filtered_metrics::Inner::Histogram(hist)) => Some((name.to_owned(), hist)),
+            _ => None,
+        })
+        .collect();
+
+    let mut histogram_titles: Vec<String> = histograms.keys().map(ToOwned::to_owned).collect();
+    histogram_titles.sort();
+    if histogram_titles.is_empty() {
+        return;
+    }
+
+    let mut histogram_table = Table::new();
+    histogram_table.set_format(*prettytable::format::consts::FORMAT_BOX_CHARS);
+
+    let mut first_row = Row::new(vec![cell!("Histograms (ms)"), cell!("sum"), cell!("count")]);
+
+    let biggest_hist_length = histograms
+        .values()
+        .map(|hist| hist.buckets.len())
+        .max()
+        .unwrap_or(0);
+
+    // 0, 1, 3, 7... in the upper row
+    for exponent in 0..biggest_hist_length {
+        first_row.add_cell(cell!(format!("{}", (1 << exponent) - 1)));
+    }
+    histogram_table.set_titles(first_row);
+
+    for title in histogram_titles {
+        if let Some(hist) = histograms.get(&title) {
+            let trimmed_name = title.strip_suffix("_hist").unwrap_or_default();
+            let mut row = Row::new(vec![
+                cell!(trimmed_name),
+                cell!(hist.sum),
+                cell!(hist.count),
+            ]);
+            for bucket in &hist.buckets {
+                row.add_cell(cell!(bucket.count));
+            }
+            histogram_table.add_row(row);
+        }
+    }
+
+    histogram_table.printstd();
+}
+
 fn print_available_metrics(available_metrics: &AvailableMetrics) -> Result<(), DisplayError> {
     println!("Available metrics on the proxy level:");
     for metric_name in &available_metrics.proxy_metrics {