feat: added gauge type & some tests

exporter/exporter.v

@@ -0,0 +1,10 @@
+module exporter
+
+import io
+import metrics { MetricsCollector }
+
+pub interface MetricsExporter {
+    load(collector MetricsCollector)
+    export_to_writer(writer io.Writer) !
+    export_to_string() string !
+}

exporter/prometheus.v

@@ -0,0 +1,4 @@
+module exporter
+
+pub struct PrometheusExporter {
+}