Automatically add custom taxonomy when meta value gets to a set number or beyond

I think the best option would be is to use your getPostViews() count and incorporate a system in there to append the trending term to the post once your counter hits 150.

Here is our worksflow

• Get the post view count (which you are already doing)

• Compare the current count to the desired 150

• Use wp_set_object_terms() to append the trending term to the post

IN CODE

(I have modified your code slightly. Also note, the code is untested)

function getPostViews()
{
    // Check if we are on a single page, bail out if not
    if ( !is_single() )
        return false;

    // Get the current post id
    $current_post_id = get_queried_object_id();

    $count_key = 'post_views_count';
    $count = get_post_meta( $current_post_id, $count_key, true );

    // Set up our function to set our term 'trending'
    if ( $count >= 150 ) {
        /**
         * First check if the term is not yet set, if not, set the term
         * I have used trending as term slug. Change as needed
         * Also, add the correct taxonomy to which trending term belongs to
         *
         * @link https://codex.wordpress.org/Function_Reference/has_term
         */
        if ( !has_term( 'trending', 'MY_TAX', $current_post_id ) ) {
            /**
             * Append the term trending to the post
             *
             * @link https://codex.wordpress.org/Function_Reference/wp_set_object_terms
             */
            wp_set_object_terms( 
                $current_post_id, // Current post ID
               'trending', // Slug of term to append
               'MY_TAX', // Taxonomy that the term trending belongs to
               true // VERY VERY IMPORTANT, Append the term to the other existing terms
            );
        }
    }

    if( !$count ){

        delete_post_meta( $current_post_id, $count_key );
        add_post_meta( $current_post_id, $count_key, '0' );

    }

    $views = ( $count ) ? $count . ' Views' : '0 View';
    return $views;

}

You can just simply use the code in your single page as follow

echo getPostViews();

and the function does the rest