Add metrics dashboard json schemas

Add metrics dashboard json schemas to be used for metrics dashboard
validation

lib/gitlab/metrics/dashboard/validator/schemas/axis.json

+{
+  "type": "object",
+  "properties": {
+    "name": { "type": "string" },
+    "format": {
+      "type": "string",
+      "default": "engineering"
+    },
+    "precision": {
+      "type": "number",
+      "default": 2
+     }
+  }
+}

lib/gitlab/metrics/dashboard/validator/schemas/dashboard.json

+{
+  "type": "object",
+  "required": ["dashboard", "panel_groups"],
+  "properties": {
+    "dashboard": { "type": "string" },
+    "panel_groups": {
+      "type": "array",
+      "items": { "$ref": "./panel_group.json" }
+    },
+    "templating": {
+      "$ref": "./templating.json"
+    },
+    "links": {
+      "type": "array",
+      "items": { "$ref": "./link.json" }
+    }
+  }
+}

lib/gitlab/metrics/dashboard/validator/schemas/link.json

+{
+  "type": "object",
+  "required": ["url"],
+  "properties": {
+    "url": { "type": "string" },
+    "title": { "type": "string" },
+    "type": {
+      "type": "string",
+      "enum": ["grafana"]
+    }
+  }
+}

lib/gitlab/metrics/dashboard/validator/schemas/metric.json

+{
+	"type": "object",
+	"required": ["unit"],
+	"oneOf": [{ "required": ["query"] }, { "required": ["query_range"] }],
+	"properties": {
+		"id": {
+			"type": "string",
+			"format": "add_to_metric_id_cache"
+		 },
+		"unit": { "type": "string" },
+		"label": { "type": "string" },
+		"query": { "type": "string" },
+		"query_range": { "type": "string" },
+		"step": { "type": "number" }
+	}
+}

lib/gitlab/metrics/dashboard/validator/schemas/panel.json

+{
+  "type": "object",
+  "required": ["title", "metrics"],
+  "properties": {
+    "type": {
+      "type": "string",
+      "enum": ["area-chart", "anomaly-chart", "bar", "column", "stacked-column", "single-stat", "heatmap"],
+      "default": "area-chart"
+    },
+    "title": { "type": "string" },
+    "y_label": { "type": "string" },
+    "y_axis": { "$ref": "./axis.json" },
+    "max_value": { "type": "number" },
+    "weight": { "type": "number" },
+    "metrics": {
+      "type": "array",
+      "items": { "$ref": "./metric.json" }
+    },
+    "links": {
+      "type": "array",
+      "items": { "$ref": "./link.json" }
+    }
+  }
+}

lib/gitlab/metrics/dashboard/validator/schemas/panel_group.json

+{
+  "type": "object",
+  "required": ["group", "panels"],
+  "properties": {
+    "group": { "type": "string" },
+    "priority": { "type": "number" },
+    "panels": {
+      "type": "array",
+      "items": { "$ref": "./panel.json" }
+    }
+  }
+}

lib/gitlab/metrics/dashboard/validator/schemas/templating.json

+{
+  "type": "object",
+  "required": ["variables"],
+  "properties": {
+    "variables": { "type": "object" }
+  }
+}