LogStash::Agent metrics after config reloading when reloading a good config does not set the last reload error

I saw this while trying to debug another intermittent failure using:

`bin/rspec ./logstash-core/spec/logstash/agent_spec.rb -e "metrics after config reloading when reloading a good config does not set the last reload error"`

```
Failures:

1) LogStash::Agent metrics after config reloading when reloading a good config does not set the last reload error
Failure/Error: output_events_map = Hash.new { |h, k| h[k] = [] }

NoMethodError:
undefined method `[]=' for #
# ./logstash-core/lib/logstash/pipeline.rb:519:in `block in output_batch'
# ./logstash-core/lib/logstash/pipeline.rb:526:in `block in output_batch'
# ./logstash-core/lib/logstash/util/wrapped_acked_queue.rb:297:in `block in each'
# ./logstash-core/lib/logstash/util/wrapped_acked_queue.rb:296:in `each'
# ./logstash-core/lib/logstash/pipeline.rb:520:in `output_batch'
# ./logstash-core/lib/logstash/pipeline.rb:480:in `worker_loop'
# ./logstash-core/lib/logstash/pipeline.rb:437:in `block in start_workers'

Finished in 7.13 seconds (files took 9.2 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./logstash-core/spec/logstash/agent_spec.rb:416 # LogStash::Agent metrics after config reloading when reloading a good config does not set the last reload error

Randomized with seed 14439
```