Doc: Add topic and expand info for in-memory queue (#13246)

Co-authored-by: Rob Bavey <rob.bavey@elastic.co>

docs/static/mem-queue.asciidoc

@@ -0,0 +1,61 @@
+[[memory-queue]]
+=== Memory queue 
+
+By default, Logstash uses in-memory bounded queues between pipeline stages (inputs → pipeline workers) to buffer events. 
+If Logstash experiences a temporary machine failure, the contents of the memory queue will be lost. 
+Temporary machine failures are scenarios where Logstash or its host machine are terminated abnormally, but are capable of being restarted. 
+
+[[mem-queue-benefits]]
+==== Benefits of memory queues
+
+The memory queue might be a good choice if you value throughput over data resiliency. 
+
+* Easier configuration
+* Easier management and administration
+* Faster throughput
+
+[[mem-queue-limitations]]
+==== Limitations of memory queues
+
+* Can lose data in abnormal termination
+* Don't do well handling sudden bursts of data, where extra capacity in needed for {ls} to catch up
+
+TIP: Consider using <<persistent-queues,persistent queues>> to avoid these limitations. 
+
+[[sizing-mem-queue]]
+==== Memory queue size
+
+Memory queue size is not configured directly.
+It is defined by the number of events, the size of which can vary greatly depending on the event payload. 
+
+The maximum number of events that can be held in each memory queue is equal to
+the value of `pipeline.batch.size` multiplied by the value of
+`pipeline.workers`.
+This value is called the "inflight count." 
+
+NOTE: Each pipeline has its own queue.
+
+See <<tuning-logstash>> for more info on the effects of adjusting `pipeline.batch.size` and `pipeline.workers`.
+
+[[mq-settings]]
+===== Settings that affect queue size
+
+These values can be configured in `logstash.yml` and `pipelines.yml`. 
+
+pipeline.batch.size::
+Number events to retrieve from inputs before sending to filters+workers
+The default is 125.
+
+pipelines.workers::
+Number of workers that will, in parallel, execute the filters+outputs stage of the pipeline.
+This value defaults to the number of the host's CPU cores.
+
+[[backpressure-mem-queue]]
+==== Back pressure
+
+When the queue is full, Logstash puts back pressure on the inputs to stall data
+flowing into Logstash. 
+This mechanism helps Logstash control the rate of data flow at the input stage
+without overwhelming outputs like Elasticsearch.
+
+Each input handles back pressure independently.