Introduces getGetLegendGraphic method

Adds the `getGetLegendGraphic` method to `ImageWMS` and `TileWMS` source.
Also adds an example and corresponding tests.

src/ol/source/ImageWMS.js

@@ -191,6 +191,40 @@ class ImageWMS extends ImageSource {
       1, sourceProjectionObj || projectionObj, baseParams);
   }
 
+  /**
+   * Return the GetLegendGraphic URL for the passed resolution.
+   * Return `undefined` if the GetLegendGraphic URL cannot be constructed.
+   * @param {number} resolution Resolution.
+   * @param {!Object} params GetLegendGraphic params. Default `FORMAT` is
+   *     `image/png`. `VERSION` should not be specified here.
+   * @return {string|undefined} GetLegendGraphic URL.
+   * @api
+   */
+  getGetLegendGraphicUrl(resolution, params) {
+    const layers = this.params_.LAYERS;
+    const isSingleLayer = !Array.isArray(layers) || this.params_['LAYERS'].length === 1;
+    if (this.url_ === undefined || !isSingleLayer) {
+      return undefined;
+    }
+
+    const mpu = this.getProjection() ? this.getProjection().getMetersPerUnit() : 1;
+    const dpi = 25.4 / 0.28;
+    const inchesPerMeter = 39.37;
+    const scale = resolution * mpu * inchesPerMeter * dpi;
+
+    const baseParams = {
+      'SERVICE': 'WMS',
+      'VERSION': DEFAULT_WMS_VERSION,
+      'REQUEST': 'GetLegendGraphic',
+      'FORMAT': 'image/png',
+      'LAYER': layers,
+      'SCALE': scale
+    };
+    assign(baseParams, params);
+
+    return appendParams(/** @type {string} */ (this.url_), baseParams);
+  }
+
   /**
    * Get the user-provided params, i.e. those passed to the constructor through
    * the "params" option, and possibly updated using the updateParams method.