Deterministic render ordering

tests/test_metric.cpp

@@ -7,7 +7,9 @@
 #include <atomic>
 #include <chrono>
 #include <cmath>
+#include <fstream>
 #include <latch>
+#include <sstream>
 #include <thread>
 #include <vector>
 
@@ -657,3 +659,98 @@ TEST_CASE("memory management") {
     CHECK(final_output.size() > 0);
   }
 }
+
+TEST_CASE("render output deterministic order golden test") {
+  // Clean slate - reset all metrics before this test
+  metric::reset_metrics_for_testing();
+
+  ArenaAllocator arena;
+
+  // Create a comprehensive set of metrics with deliberate ordering
+  // to test deterministic output
+
+  // Create counters with different family names and labels
+  auto z_counter_family =
+      metric::create_counter("z_last_counter", "Last counter alphabetically");
+  auto z_counter =
+      z_counter_family.create({{"method", "POST"}, {"handler", "api"}});
+  z_counter.inc(42.0);
+
+  auto a_counter_family =
+      metric::create_counter("a_first_counter", "First counter alphabetically");
+  auto a_counter1 = a_counter_family.create({{"status", "200"}});
+  auto a_counter2 = a_counter_family.create(
+      {{"method", "GET"}}); // Should come before status lexicographically
+  a_counter1.inc(100.0);
+  a_counter2.inc(200.0);
+
+  // Create gauges with different orderings
+  auto m_gauge_family = metric::create_gauge("m_middle_gauge", "Middle gauge");
+  auto m_gauge = m_gauge_family.create({{"type", "memory"}});
+  m_gauge.set(1024.0);
+
+  auto b_gauge_family = metric::create_gauge("b_second_gauge", "Second gauge");
+  auto b_gauge = b_gauge_family.create({{"region", "us-west"}});
+  b_gauge.set(256.0);
+
+  // Create histograms
+  auto x_hist_family = metric::create_histogram("x_histogram", "Test histogram",
+                                                {0.1, 0.5, 1.0});
+  auto x_hist = x_hist_family.create({{"endpoint", "/api/v1"}});
+  x_hist.observe(0.25);
+  x_hist.observe(0.75);
+
+  // Add some callbacks to test callback ordering
+  a_counter_family.register_callback({{"callback", "test"}},
+                                     []() { return 123.0; });
+  m_gauge_family.register_callback({{"callback", "dynamic"}},
+                                   []() { return 456.0; });
+
+  // Render the metrics
+  auto output = metric::render(arena);
+
+  // Concatenate all output into a single string
+  std::ostringstream oss;
+  for (const auto &line : output) {
+    oss << line;
+  }
+  std::string actual_output = oss.str();
+
+  // Define expected golden output - this represents the exact expected
+  // deterministic order
+  std::string expected_golden =
+      "# HELP a_first_counter First counter alphabetically\n"
+      "# TYPE a_first_counter counter\n"
+      "a_first_counter{callback=\"test\"} 123\n"
+      "a_first_counter{method=\"GET\"} 200\n"
+      "a_first_counter{status=\"200\"} 100\n"
+      "# HELP z_last_counter Last counter alphabetically\n"
+      "# TYPE z_last_counter counter\n"
+      "z_last_counter{handler=\"api\",method=\"POST\"} 42\n"
+      "# HELP b_second_gauge Second gauge\n"
+      "# TYPE b_second_gauge gauge\n"
+      "b_second_gauge{region=\"us-west\"} 256\n"
+      "# HELP m_middle_gauge Middle gauge\n"
+      "# TYPE m_middle_gauge gauge\n"
+      "m_middle_gauge{callback=\"dynamic\"} 456\n"
+      "m_middle_gauge{type=\"memory\"} 1024\n"
+      "# HELP x_histogram Test histogram\n"
+      "# TYPE x_histogram histogram\n"
+      "x_histogram_bucket{endpoint=\"/api/v1\",le=\"0.1\"} 0\n"
+      "x_histogram_bucket{endpoint=\"/api/v1\",le=\"0.5\"} 1\n"
+      "x_histogram_bucket{endpoint=\"/api/v1\",le=\"1.0\"} 2\n"
+      "x_histogram_bucket{endpoint=\"/api/v1\",le=\"+Inf\"} 2\n"
+      "x_histogram_sum{endpoint=\"/api/v1\"} 1\n"
+      "x_histogram_count{endpoint=\"/api/v1\"} 2\n";
+
+  // Check if output matches golden file
+  if (actual_output != expected_golden) {
+    MESSAGE("Render output does not match expected golden output.");
+    MESSAGE("This indicates the deterministic ordering has changed.");
+    MESSAGE("Expected output:\n" << expected_golden);
+    MESSAGE("Actual output:\n" << actual_output);
+    CHECK(false); // Force test failure
+  } else {
+    CHECK(true); // Test passes
+  }
+}