Add 4 missing duration and queue size metrics

[rifelpet]

This adds the following metrics:

logstash_node_queue_size_bytes{pipeline="main"} 1.4235844e+07
logstash_node_pipeline_queue_push_duration_seconds_total{pipeline="main"} 382
logstash_node_plugin_queue_push_duration_seconds_total{pipeline="main",plugin="beats",plugin_id="beats",plugin_type="input"} 0
logstash_node_plugin_duration_seconds_total{pipeline="main",plugin="elasticsearch",plugin_id="es",plugin_type="output"} 67

This resolves #17

[tivvit]

I think this should be merged, the metrics are missing

[tivvit]

I tested the code and it works

[danielmotaleite]

This repo is dead, use the below fork #35

[rifelpet]

closing because the fork mentioned above includes this PR and we'll be switching to that.