Improve validation

Signed-off-by: David Arendsen <darendsen@gamepoint.com>

src/Functions/Measurement.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+class Measurement extends Base
+{
+    /**
+     * @var string $measurement
+     */
+    private $measurement;
+
+    public function __construct(string $measurement)
+    {
+        $this->measurement = $measurement;
+    }
+
+    public function __toString()
+    {
+        return '|> filter(fn: (r) => r._measurement == "' . (string)$this->measurement . '") ';
+    }
+}