This is an example implementation of the patch for https://core.trac.wordpress.org/ticket/37549

example.php

<?php

add_filter( 'wp_generate_attachment_metadata', __NAMESPACE__ . '\\update_attachment_metadata', 10, 2 );

function update_attachment_metadata( $data, $attachment_id ) {
	$dimensions = false;

	$post = get_post( $attachment_id );
	$file = wp_get_attachment_url( $attachment_id );
	$path = get_attached_file( $attachment_id );

	// If we're dealing with an SVG -- this is probably empty.
	// But, in the future it might not be and we should preserve that date.
	$data = wp_get_attachment_metadata( $attachment_id );

	if( $post->post_mime_type !== 'image/svg+xml' ) {
		return false;
	}

	$xml = simplexml_load_file( $file );
	$attributes = $xml->attributes();

	// Yay we have a proper viewport!
	// This is the ideal way to get dimensions for the SVG.
	if( isset( $attributes->width ) && isset( $attributes->height ) ) {
		$dimensions = array(
			'width' => $attributes->width,
			'height' => $attributes->height
		);
	}

	// Whelp... we don't have a viewport... time to hacky hacky?
	// The viewBox attribute allows to specify that a given set of graphics
	// stretch to fit a particular container element.
	// The value of the viewBox attribute is a list of four numbers
	// min-x, min-y, width and height, separated by whitespace and/or a comma,
	// which specify a rectangle in user space which should be mapped to the
	// bounds of the viewport established by the given element, taking into
	// account attribute preserveAspectRatio.
	//
	// I do not believe it's a required attribute though...
	//
	// Source: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/viewBox
	if( ! $dimensions ) {
		$viewbox = explode( ' ', $attributes->viewBox );

		if( isset( $viewbox[ 2 ] ) && isset( $viewbox[ 3 ] ) ) {
			$dimensions = array(
				'width' => $viewbox[ 2 ],
				'height' => $viewbox[ 3 ]
			);
		}
	}

	if( isset( $dimensions[ 'width' ] ) && isset( $dimensions[ 'height' ] ) ) {
		if( is_array( $data ) ) {
			$data = array_merge( $data, $dimensions );
		} else {
			$data = $dimensions;
		}

	}

	return $data;

}

?>