Refactor docs; Add 'ipng_source_tag', add udp listener for nginx-ipng-stats plugin

cmd/frontend/filter.go

@@ -146,8 +146,13 @@ func applyTerm(term string, fs *filterState) error {
 			return fmt.Errorf("invalid asn expression %q", expr)
 		}
 		fs.ASN = expr
+	case "source_tag":
+		if op != "=" {
+			return fmt.Errorf("source_tag only supports =, not %q", op)
+		}
+		fs.SourceTag = value
 	default:
-		return fmt.Errorf("unknown field %q; valid: status, website, uri, prefix, is_tor, asn", field)
+		return fmt.Errorf("unknown field %q; valid: status, website, uri, prefix, is_tor, asn, source_tag", field)
 	}
 	return nil
 }
@@ -196,6 +201,9 @@ func FilterExprString(f filterState) string {
 	if f.ASN != "" {
 		parts = append(parts, asnTermStr(f.ASN))
 	}
+	if f.SourceTag != "" {
+		parts = append(parts, "source_tag="+quoteMaybe(f.SourceTag))
+	}
 	return strings.Join(parts, " AND ")
 }