Code cleanup for the collector observer

We have more the responsability of watching the collector inside the
input itself, this feature might come back when we have a new execution
model that can be improved in watching metrics. But this would require
more granular watchers.

No tests were affected by this changes since the code that required that
features was already removed.

Fixes: #6447

Fixes #6456

logstash-core/lib/logstash/agent.rb

@@ -130,7 +130,6 @@ class LogStash::Agent
   end
 
   def stop_collecting_metrics
-    @collector.stop
     @periodic_pollers.stop
   end
 

logstash-core/lib/logstash/instrument/collector.rb

@@ -11,12 +11,8 @@ module LogStash module Instrument
   # The Collector is the single point of reference for all
   # the metrics collection inside logstash, the metrics library will make
   # direct calls to this class.
-  #
-  # This class is an observable responsable of periodically emitting view of the system
-  # to other components like the internal metrics pipelines.
   class Collector
     include LogStash::Util::Loggable
-    include Observable
 
     SNAPSHOT_ROTATION_TIME_SECS = 1 # seconds
     SNAPSHOT_ROTATION_TIMEOUT_INTERVAL_SECS = 10 * 60 # seconds
@@ -26,7 +22,6 @@ module LogStash module Instrument
     def initialize
       @metric_store = MetricStore.new
       @agent = nil
-      start_periodic_snapshotting
     end
 
     # The metric library will call this unique interface
@@ -43,8 +38,6 @@ module LogStash module Instrument
         end
 
         metric.execute(*metric_type_params)
-
-        changed # we had changes coming in so we can notify the observers
       rescue MetricStore::NamespacesExpectedError => e
         logger.error("Collector: Cannot record metric", :exception => e)
       rescue NameError => e
@@ -58,51 +51,13 @@ module LogStash module Instrument
       end
     end
 
-    # Monitor the `Concurrent::TimerTask` this update is triggered on every successful or not
-    # run of the task, TimerTask implement Observable and the collector acts as
-    # the observer and will keep track if something went wrong in the execution.
-    #
-    # @param [Time] Time of execution
-    # @param [result] Result of the execution
-    # @param [Exception] Exception
-    def update(time_of_execution, result, exception)
-      return true if exception.nil?
-      logger.error("Collector: Something went wrong went sending data to the observers",
-                   :execution_time => time_of_execution,
-                   :result => result,
-                   :exception => exception.class.name)
-    end
-
     # Snapshot the current Metric Store and return it immediately,
     # This is useful if you want to get access to the current metric store without
     # waiting for a periodic call.
     #
     # @return [LogStash::Instrument::MetricStore]
     def snapshot_metric
-      Snapshot.new(@metric_store)
-    end
-
-    # Configure and start the periodic task for snapshotting the `MetricStore`
-    def start_periodic_snapshotting
-      @snapshot_task = Concurrent::TimerTask.new { publish_snapshot }
-      @snapshot_task.execution_interval = SNAPSHOT_ROTATION_TIME_SECS
-      @snapshot_task.timeout_interval = SNAPSHOT_ROTATION_TIMEOUT_INTERVAL_SECS
-      @snapshot_task.add_observer(self)
-      @snapshot_task.execute
-    end
-
-    def stop
-      @snapshot_task.shutdown
-    end
-
-    # Create a snapshot of the MetricStore and send it to to the registered observers
-    # The observer will receive the following signature in the update methode.
-    #
-    # `#update(created_at, metric_store)`
-    def publish_snapshot
-      created_at = Time.now
-      logger.debug("Collector: Sending snapshot to observers", :created_at => created_at) if logger.debug?
-      notify_observers(snapshot_metric)
+      Snapshot.new(@metric_store.dup)
     end
 
     def clear(keypath)