Marker label can be bool

[mrwonko]

TileDefMarker.Label is a *string, but for one of my screenboards Datadog returns true, which results in this error:

json: cannot unmarshal bool into Go struct field TileDefMarker.label of type string

Here's an excerpt of the screenboard that caused this:

{
	"board_title": "anonymized",
	"read_only": false,
	"author_info": {
		"author_name": "anonymous"
	},
	"description": "",
	"board_bgtype": "board_graph",
	"created": "2019-02-19T13:47:39.808958+00:00",
	"created_by": {
		"disabled": false,
		"handle": "anonymous",
		"name": "anonymous",
		"is_admin": false,
		"role": null,
		"access_role": "st",
		"verified": true,
		"email": "anonymous",
		"icon": "https://secure.gravatar.com/avatar/42?s=48&d=retro"
	},
	"new_id": "ano-nym-ous",
	"modified": "2019-02-27T15:50:13.939611+00:00",
	"originalHeight": 80,
	"height": 80,
	"width": "100%",
	"isIntegration": false,
	"disableEditing": false,
	"originalWidth": "100%",
	"widgets": [
		{
			"board_id": 585431,
			"globalTimeframe": {
				"isLive": true,
				"start": 1551278820000,
				"end": 1551282420000
			},
			"title_align": "left",
			"title_size": 16,
			"title": true,
			"generated_title": "anonymized",
			"title_text": "anonymized",
			"height": 13,
			"tile_def": {
				"viz": "timeseries",
				"requests": [
					{
						"q": "sum:anonymized.as_count()",
						"aggregator": "avg",
						"style": {
							"width": "normal",
							"palette": "dog_classic",
							"type": "solid"
						},
						"type": "line",
						"conditional_formats": []
					}
				],
				"autoscale": true,
				"markers": [
					{
						"type": "info bold",
						"val": "0",
						"value": "y = 0",
						"label": true
					}
				]
			},
			"width": 47,
			"time": {},
			"y": 2,
			"x": 48,
			"legend_size": "0",
			"add_timeframe": true,
			"type": "timeseries",
			"legend": false,
			"isShared": false
		}
	],
	"disableCog": false,
	"id": 42,
	"isShared": false
}