test: fix driver activity period

[Watson1978]

This PR prevent to shut down the driver used in subsequent tests.
When we use timer plugin helper or other components that depend on the event loop, the driver does not operate as expected if it is shut down.

If you add timer plugin helper, you can confirm this behavior.

diff --git a/lib/fluent/plugin/filter_prometheus.rb b/lib/fluent/plugin/filter_prometheus.rb
index ccdfe78..ab3a889 100644
--- a/lib/fluent/plugin/filter_prometheus.rb
+++ b/lib/fluent/plugin/filter_prometheus.rb
@@ -7,6 +7,8 @@ class PrometheusFilter < Fluent::Plugin::Filter
     include Fluent::Plugin::PrometheusLabelParser
     include Fluent::Plugin::Prometheus
 
+    helpers :timer
+
     def initialize
       super
       @registry = ::Prometheus::Client.registry

[daipom]

Thanks!