Add single_stat chart component to the monitoring bundle

This merge requests just adds the component without integrating it
to the dashboard, it adds tests as well

app/assets/javascripts/monitoring/components/charts/single_stat.vue

+<script>
+import { GlSingleStat } from '@gitlab/ui/dist/charts';
+
+export default {
+  components: {
+    GlSingleStat,
+  },
+  inheritAttrs: false,
+  props: {
+    title: {
+      type: String,
+      required: true,
+    },
+    value: {
+      type: Number,
+      required: true,
+    },
+    unit: {
+      type: String,
+      required: true,
+    },
+  },
+  computed: {
+    valueWithUnit() {
+      return `${this.value}${this.unit}`;
+    },
+  },
+};
+</script>
+<template>
+  <div class="prometheus-graph col-12 col-lg-6">
+    <div class="prometheus-graph-header">
+      <h5 ref="graphTitle" class="prometheus-graph-title">{{ title }}</h5>
+    </div>
+    <gl-single-stat :value="valueWithUnit" :title="title" variant="success" />
+  </div>
+</template>

spec/javascripts/monitoring/charts/single_stat_spec.js

+import { shallowMount } from '@vue/test-utils';
+import SingleStatChart from '~/monitoring/components/charts/single_stat.vue';
+
+describe('Single Stat Chart component', () => {
+  let singleStatChart;
+
+  beforeEach(() => {
+    singleStatChart = shallowMount(SingleStatChart, {
+      propsData: {
+        title: 'Time to render',
+        value: 1,
+        unit: 'sec',
+      },
+    });
+  });
+
+  afterEach(() => {
+    singleStatChart.destroy();
+  });
+
+  describe('computed', () => {
+    describe('valueWithUnit', () => {
+      it('should interpolate the value and unit props', () => {
+        expect(singleStatChart.vm.valueWithUnit).toBe('1sec');
+      });
+    });
+  });
+});