fix: order Prometheus histogram metrics correctly

src/bentoml/_internal/server/metrics/prometheus.py

@@ -2,6 +2,7 @@
 
 import logging
 import os
+import re
 import typing as t
 from functools import partial
 from typing import TYPE_CHECKING
@@ -101,9 +102,107 @@ def generate_latest(self):
         if self.multiproc:
             registry = self.prometheus_client.CollectorRegistry()
             self.prometheus_client.multiprocess.MultiProcessCollector(registry)
-            return self.prometheus_client.generate_latest(registry)
+            raw_output = self.prometheus_client.generate_latest(registry)
+            return self._fix_histogram_ordering(raw_output)
         else:
-            return self.prometheus_client.generate_latest()
+            raw_output = self.prometheus_client.generate_latest()
+            return self._fix_histogram_ordering(raw_output)
+
+    def _fix_histogram_ordering(self, prometheus_output: bytes) -> bytes:
+        """
+        Fix histogram metric ordering to comply with Prometheus text format specification.
+
+        The Prometheus format requires histogram metrics to be grouped by metric name with:
+        1. All _bucket metrics for a histogram (in ascending order of 'le' values)
+        2. Followed by _count metric
+        3. Followed by _sum metric
+
+        Args:
+            prometheus_output: Raw Prometheus format output
+
+        Returns:
+            Properly ordered Prometheus format output
+        """
+        lines = prometheus_output.decode("utf-8").strip().split("\n")
+
+        # Separate comments/help lines from metric lines
+        comment_lines = []
+        metric_lines = []
+
+        for line in lines:
+            if line.startswith("#") or line.strip() == "":
+                comment_lines.append(line)
+            else:
+                metric_lines.append(line)
+
+        # Group metrics by base name (without _bucket, _count, _sum suffixes)
+        metrics_by_base = {}
+        non_histogram_metrics = []
+
+        for line in metric_lines:
+            if not line.strip():
+                continue
+
+            # Extract metric name (everything before the first space or '{')
+            if "{" in line:
+                metric_name = line.split("{")[0]
+            else:
+                metric_name = line.split(" ")[0]
+
+            # Check if this is a histogram metric
+            if metric_name.endswith("_bucket"):
+                base_name = metric_name[:-7]  # Remove '_bucket'
+                if base_name not in metrics_by_base:
+                    metrics_by_base[base_name] = {"bucket": [], "count": [], "sum": []}
+                metrics_by_base[base_name]["bucket"].append(line)
+            elif metric_name.endswith("_count"):
+                base_name = metric_name[:-6]  # Remove '_count'
+                if base_name not in metrics_by_base:
+                    metrics_by_base[base_name] = {"bucket": [], "count": [], "sum": []}
+                metrics_by_base[base_name]["count"].append(line)
+            elif metric_name.endswith("_sum"):
+                base_name = metric_name[:-4]  # Remove '_sum'
+                if base_name not in metrics_by_base:
+                    metrics_by_base[base_name] = {"bucket": [], "count": [], "sum": []}
+                metrics_by_base[base_name]["sum"].append(line)
+            else:
+                non_histogram_metrics.append(line)
+
+        # Function to extract 'le' value for bucket sorting
+        def extract_le_value(bucket_line: str) -> float:
+            try:
+                # Find le="value" in the line
+                match = re.search(r'le="([^"]+)"', bucket_line)
+                if match:
+                    le_val = match.group(1)
+                    if le_val == "+Inf":
+                        return float("inf")
+                    return float(le_val)
+                return float("inf")  # Default if parsing fails
+            except (ValueError, TypeError):
+                return float("inf")
+
+        # Rebuild the output with proper ordering
+        result_lines = comment_lines.copy()
+
+        # Add non-histogram metrics first
+        result_lines.extend(non_histogram_metrics)
+
+        # Add histogram metrics in proper order
+        for base_name in sorted(metrics_by_base.keys()):
+            hist_data = metrics_by_base[base_name]
+
+            # Sort buckets by 'le' value in ascending order
+            sorted_buckets = sorted(hist_data["bucket"], key=extract_le_value)
+            result_lines.extend(sorted_buckets)
+
+            # Add count metrics
+            result_lines.extend(hist_data["count"])
+
+            # Add sum metrics
+            result_lines.extend(hist_data["sum"])
+
+        return "\n".join(result_lines).encode("utf-8")
 
     def text_string_to_metric_families(self) -> t.Generator[Metric, None, None]:
         yield from self.prometheus_client.parser.text_string_to_metric_families(