Tracer plugins are no longer real plugins

Tracer is a low-level module. Having tracer plugins implemented as real Drupal plugins causes more problems than benefits.

Previously, in settings.php, you have to set the plugin ID for the tracer plugin to use:

$settings['tracer_plugin'] =  'stopwatch_tracer';

Now, this setting expects the FQCN of the class implementing the plugin:

$settings['tracer_plugin'] = \Drupal\webprofiler\Plugin\Tracer\StopwatchTracer::class;