fix(scripts/metricsdocgen): shush the prometheus scanner in CI (#23642)

- Suppress informational `log.Printf` messages from the metrics scanner
when stdout is not a TTY (i.e. piped via `atomic_write` in `make gen` or
CI)
- Genuine warnings (`warnf`) still print unconditionally so real
problems remain visible
- `log.Fatalf` for fatal errors is unchanged

> 🤖 Created by Coder Agents and reviewed by a human

scripts/metricsdocgen/scanner/scanner.go

@@ -20,6 +20,7 @@ import (
 	"sort"
 	"strings"
 
+	"golang.org/x/term"
 	"golang.org/x/xerrors"
 )
 
@@ -80,6 +81,26 @@ type declarations struct {
 // constants from later directories won't be available when scanning earlier ones.
 var packageDeclarations = make(map[string]map[string]string)
 
+// verbose controls whether informational messages are printed to
+// stderr. It is true when stdout is a terminal (interactive use)
+// and false when stdout is piped (e.g. via atomic_write in make).
+var verbose = term.IsTerminal(int(os.Stdout.Fd()))
+
+// logf prints an informational message to stderr only when running
+// interactively. Use this for progress and debug output that is
+// not actionable.
+func logf(format string, args ...any) {
+	if verbose {
+		log.Printf(format, args...)
+	}
+}
+
+// warnf prints a warning to stderr unconditionally. Use this for
+// messages about real problems that a developer should investigate.
+func warnf(format string, args ...any) {
+	log.Printf("WARNING: "+format, args...)
+}
+
 func main() {
 	metrics, err := scanAllDirs()
 	if err != nil {
@@ -103,7 +124,7 @@ func main() {
 
 	writeMetrics(metrics, os.Stdout)
 
-	log.Printf("Successfully parsed %d metrics", len(metrics))
+	logf("Successfully parsed %d metrics", len(metrics))
 }
 
 // scanAllDirs scans all configured directories for metric definitions.
@@ -116,7 +137,7 @@ func scanAllDirs() ([]Metric, error) {
 			return nil, xerrors.Errorf("scanning %s: %w", dir, err)
 		}
 
-		log.Printf("scanning %s: found %d metrics", dir, len(metrics))
+		logf("scanning %s: found %d metrics", dir, len(metrics))
 		allMetrics = append(allMetrics, metrics...)
 	}
 
@@ -155,7 +176,7 @@ func scanDirectory(root string) ([]Metric, error) {
 		}
 
 		if len(fileMetrics) > 0 {
-			log.Printf("scanning %s: found %d metrics", path, len(fileMetrics))
+			logf("scanning %s: found %d metrics", path, len(fileMetrics))
 		}
 		metrics = append(metrics, fileMetrics...)
 
@@ -191,7 +212,7 @@ func scanFile(path string) ([]Metric, error) {
 		metric, ok := extractMetricFromCall(call, decls)
 		if ok {
 			if metric.Help == "" {
-				log.Printf("WARNING: metric %q has no HELP description, skipping", metric.Name)
+				warnf("metric %q has no HELP description, skipping", metric.Name)
 				// Skip metrics without descriptions, they should be fixed in the source code
 				// or added to the static metrics file with a manual description.
 				return true
@@ -392,7 +413,7 @@ func extractNewDescMetric(call *ast.CallExpr, decls declarations) (Metric, bool)
 	// Extract name (first argument).
 	name := resolveStringExpr(call.Args[0], decls)
 	if name == "" {
-		log.Printf("extractNewDescMetric: skipping prometheus.NewDesc() call: could not resolve metric name")
+		warnf("extractNewDescMetric: skipping prometheus.NewDesc() call: could not resolve metric name")
 		return Metric{}, false
 	}
 
@@ -542,7 +563,7 @@ func extractOptsMetric(call *ast.CallExpr, decls declarations) (Metric, bool) {
 	// Extract metric info from the Opts struct.
 	opts, ok := extractOpts(call.Args[0], decls)
 	if !ok {
-		log.Printf("extractOptsMetric: skipping prometheus.%s() call: could not extract opts", funcName)
+		warnf("extractOptsMetric: skipping prometheus.%s() call: could not extract opts", funcName)
 		return Metric{}, false
 	}
 
@@ -555,7 +576,7 @@ func extractOptsMetric(call *ast.CallExpr, decls declarations) (Metric, bool) {
 	// Build the full metric name.
 	name := buildMetricName(opts.Namespace, opts.Subsystem, opts.Name)
 	if name == "" {
-		log.Printf("extractOptsMetric: skipping prometheus.%s() call: could not build metric name", funcName)
+		warnf("extractOptsMetric: skipping prometheus.%s() call: could not build metric name", funcName)
 		return Metric{}, false
 	}
 
@@ -627,7 +648,7 @@ func extractPromautoMetric(call *ast.CallExpr, decls declarations) (Metric, bool
 	// Extract metric info from the Opts struct.
 	opts, ok := extractOpts(call.Args[0], decls)
 	if !ok {
-		log.Printf("extractPromautoMetric: skipping promauto.%s() call: could not extract opts", funcName)
+		warnf("extractPromautoMetric: skipping promauto.%s() call: could not extract opts", funcName)
 		return Metric{}, false
 	}
 
@@ -640,7 +661,7 @@ func extractPromautoMetric(call *ast.CallExpr, decls declarations) (Metric, bool
 	// Build the full metric name.
 	name := buildMetricName(opts.Namespace, opts.Subsystem, opts.Name)
 	if name == "" {
-		log.Printf("extractPromautoMetric: skipping promauto.%s() call: could not build metric name", funcName)
+		warnf("extractPromautoMetric: skipping promauto.%s() call: could not build metric name", funcName)
 		return Metric{}, false
 	}