Fixing Event Log file cleanup issue

[khushbr]

Is your feature request related to a problem?
Issue : #26
Previous PR [now closed] : opensearch-project/performance-analyzer#36
PA side PR : opensearch-project/performance-analyzer#36

Describe the solution you are proposing

1. The solution removes the 'MetricsPurgeActivity' collector and moves the responsibility for event log file cleanup to 'EventLogQueueProcessor.' The queue processor first invokes the deleteFiles() [taken from MetricsPurgeActivity] to clean up the old event log files and then writes the latest event log file. This ensures that we never run into issue of lingering files as the 'EventLogQueueProcessor' will first perform the cleanup before writing new files.
2. Adding additional metrics 'EVENT_LOG_FILES_DELETION_TIME', 'EVENT_LOG_FILES_DELETED', 'METRICS_WRITE_ERROR', 'METRICS_REMOVE_ERROR' and 'METRICS_REMOVE_FAILURE'
3. Refactoring MetricConfig class

Describe alternatives you've considered
Another approach was to launch a new thread and invoke 'MetricsPurgeActivity' within it. We will again run into the same issue if this thread dies, thus to keep the cleanup and write within same thread was better.

Testing
Tested by spinning up a docker container. Manually copied 100 dummy files to /dev/shm/performanceanalyzer/.
Enabled DEBUG logs to verify cleanup is working as expected

[2021-07-01T20:30:52,950][DEBUG][c.a.o.e.p.r.EventLogFileHandler] Starting to delete old writer files
[2021-07-01T20:30:52,950][DEBUG][c.a.o.e.p.r.EventLogFileHandler] Files discovered 169
[2021-07-01T20:30:52,977][DEBUG][c.a.o.e.p.r.EventLogFileHandler] '153' Old writer files cleaned up.

Metrics:

Metrics=EventLogFilesDeletionTime=27.0 millis aggr|MEAN,EventLogFilesDeletionTime=27 millis 
aggr|MAX,EventLogFilesDeletionTime=27 millis aggr|SUM,EventLogFilesDeleted=153 count 
aggr|SUM,EventLogFilesDeleted=153 count aggr|MAX

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