Move node-mixin into docs directory

Signed-off-by: beorn7 <beorn@grafana.com>

docs/node-mixin/lib/promgrafonnet/numbersinglestat.libsonnet

@@ -0,0 +1,48 @@
+local grafana = import 'grafonnet/grafana.libsonnet';
+local singlestat = grafana.singlestat;
+local prometheus = grafana.prometheus;
+
+{
+  new(title, query)::
+    singlestat.new(
+      title,
+      datasource='prometheus',
+      span=3,
+      valueName='current',
+      valueMaps=[
+        {
+          op: '=',
+          text: '0',
+          value: 'null',
+        },
+      ],
+    )
+    .addTarget(
+      prometheus.target(
+        query
+      )
+    ) + {
+      withTextNullValue(text):: self {
+        valueMaps: [
+          {
+            op: '=',
+            text: text,
+            value: 'null',
+          },
+        ],
+      },
+      withSpanSize(size):: self {
+        span: size,
+      },
+      withPostfix(postfix):: self {
+        postfix: postfix,
+      },
+      withSparkline():: self {
+        sparkline: {
+          show: true,
+          lineColor: 'rgb(31, 120, 193)',
+          fillColor: 'rgba(31, 118, 189, 0.18)',
+        },
+      },
+    },
+}