Add a new "Membership Level" dimension to track via MonsterInsights.

pmpro_monsterinsights_level_custom_dimension.php

<?php
/**
 * Add a new "Membership Level" Custom Dimension to your Google Analytics tracking code.
 *
 * Requires Paid Memberships Pro, MonsterInsights Pro and the MonsterInsights - Dimensions Addon.
 */

/**
 * Filter for adding the new dimension as under Insights > Settings > Dimensions
*/
function my_pmpro_membership_level_custom_dimension( $custom_dimensions ) {
	if( defined( 'PMPRO_VERSION' ) ) {
		$my_custom_dimensions = array(
			'membership_level' => array(
				'title'   => __( 'Membership Level', 'paid-memberships-pro' ),
				'label'   => __( 'The membership level of the user', 'paid-memberships-pro' ),
				'enabled' => true,
				'metric'  => 'sessions',
			),
		);
		$custom_dimensions = array_merge( $custom_dimensions, $my_custom_dimensions );
	}
	return $custom_dimensions;
}
add_filter( 'monsterinsights_available_custom_dimensions', 'my_pmpro_membership_level_custom_dimension' );

/**
 * Filter for adding the tracking code for the "Membership Level" Custom Dimension.
*/
function my_pmpro_monsterinsights_frontend_tracking_options_analytics_before_pageview( $options ) {
	if ( defined( 'PMPRO_VERSION' ) ) {
		
		// Get all active Custom Dimensions in MonsterInsights. 
		$dimensions = monsterinsights_get_option( 'custom_dimensions', array() );
		
		if ( ! empty( $dimensions ) && is_array( $dimensions ) ) {
			foreach ( $dimensions as $dimension ) {
				// Set the tracking value for membership_level Custom Dimension in MonsterInsights.
				if ( ( $dimension['type'] === 'membership_level' ) && ! empty( $dimension['id'] ) ) {
					$type = $dimension['type'];
                			$id = $dimension['id'];
					if ( is_user_logged_in() ) {
						// Get the current users's membership level ID. 
						$current_user_membership_level = pmpro_getMembershipLevelForUser( get_current_user_id() );
						$value = $current_user_membership_level->ID;
					} else {
						// Set the tracked membership level ID to no_level.
						$value = 'no_level';
					}

					// Set the Custom Dimension's value to the output.
					if ( ! empty( $value ) ) {
                 	   			$options[ 'dimension' . $id ] = "'set', 'dimension" . absint( $id ) . "', '" . esc_js( addslashes( $value ) ) . "'";
		                	}
				}
			}
		}
	}
	return $options;
}
add_filter( 'monsterinsights_frontend_tracking_options_analytics_before_pageview', 'my_pmpro_monsterinsights_frontend_tracking_options_analytics_before_pageview' );

This recipe is included in the blog post on "Putting Google Analytics to work for your Membership Site" at Paid Memberships Pro here: https://www.paidmembershipspro.com/putting-google-analytics-to-work-for-your-membership-site/