metrics: Prometheus text exposition for /metrics

/metrics was Prometheus-ish name/value text with a custom banner and
text/plain Content-Type, so strict scrapers could fail.

Emit Prometheus text 0.0.4 (# HELP, # TYPE, counter|gauge) with stable
metric names, and set Content-Type to text/plain; version=0.0.4; charset=utf-8.

steamcache/range_test.go

@@ -313,6 +313,18 @@ func TestRangeMetricsWriteText(t *testing.T) {
 	if !strings.Contains(text, "range_upstream 1\n") {
 		t.Errorf("/metrics missing 'range_upstream 1' line:\n%s", text)
 	}
+	if ct := resp.Header.Get("Content-Type"); ct != "text/plain; version=0.0.4; charset=utf-8" {
+		t.Errorf("/metrics Content-Type=%q, want Prometheus text 0.0.4", ct)
+	}
+	if !strings.Contains(text, "# HELP range_cache ") {
+		t.Errorf("/metrics missing # HELP range_cache:\n%s", text)
+	}
+	if !strings.Contains(text, "# TYPE range_cache counter") {
+		t.Errorf("/metrics missing # TYPE range_cache counter:\n%s", text)
+	}
+	if !strings.Contains(text, "# TYPE range_upstream counter") {
+		t.Errorf("/metrics missing # TYPE range_upstream counter:\n%s", text)
+	}
 }
 
 // TestStreamCachedResponseRange206 is a focused unit test for streamCachedResponse: