Retrieve last item of a list

Hello

Is your feature request related to a problem?

It may already be possible, but I would like a mutate to retrieve last item of a list.
For example:

{
  "my-list": [
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  ]
}

Would become:

{
  ...
  "lastkey": "value3"
}

Additional context

I tried with add_entries and using -1 to get it:

  processor:
    - add_entries:
        entries:
          - key: "my-key"
            value_expression: /my-list/-1

But it didn't work I had the following error:

2024-03-28T16:30:37,711 [waf-log-pipeline-processor-worker-1-thread-1] ERROR org.opensearch.dataprepper.expression.ParseTreeEvaluator - Unable to evaluate event
org.opensearch.dataprepper.expression.ExpressionEvaluationException: Unable to evaluate the part of input statement: /my-list/-1
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.exitEveryRule(ParseTreeEvaluatorListener.java:91) ~[data-prepper-expression-2.7.0.jar:?]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.exitRule(ParseTreeWalker.java:63) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:38) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:37) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:17) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.GenericExpressionEvaluator.evaluate(GenericExpressionEvaluator.java:39) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.plugins.processor.mutateevent.AddEntryProcessor.doExecute(AddEntryProcessor.java:61) ~[mutate-event-processors-2.7.0.jar:?]
	at org.opensearch.dataprepper.model.processor.AbstractProcessor.lambda$execute$0(AbstractProcessor.java:54) ~[data-prepper-api-2.7.0.jar:?]
	at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:69) [micrometer-core-1.11.5.jar:1.11.5]
	at org.opensearch.dataprepper.model.processor.AbstractProcessor.execute(AbstractProcessor.java:54) [data-prepper-api-2.7.0.jar:?]
	at org.opensearch.dataprepper.pipeline.ProcessWorker.doRun(ProcessWorker.java:135) [data-prepper-core-2.7.0.jar:?]
	at org.opensearch.dataprepper.pipeline.ProcessWorker.run(ProcessWorker.java:61) [data-prepper-core-2.7.0.jar:?]
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539) [?:?]
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
	at java.base/java.lang.Thread.run(Thread.java:840) [?:?]
Caused by: java.lang.IllegalArgumentException: DIVIDE requires left operand to be either Float or Integer.
	at org.opensearch.dataprepper.expression.ArithmeticBinaryOperator.evaluate(ArithmeticBinaryOperator.java:46) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ArithmeticBinaryOperator.evaluate(ArithmeticBinaryOperator.java:16) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.performSingleOperation(ParseTreeEvaluatorListener.java:104) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.exitEveryRule(ParseTreeEvaluatorListener.java:88) ~[data-prepper-expression-2.7.0.jar:?]
	... 18 more
2024-03-28T16:30:37,717 [waf-log-pipeline-processor-worker-1-thread-1] ERROR org.opensearch.dataprepper.plugins.processor.mutateevent.AddEntryProcessor - Error adding entry to record [org.opensearch.dataprepper.model.log.JacksonLog@50405800] with key [my-key], metadataKey [null], value_expression [/my-list/-1] format [null], value [null]
org.opensearch.dataprepper.expression.ExpressionEvaluationException: Unable to evaluate statement "/my-list/-1"
	at org.opensearch.dataprepper.expression.GenericExpressionEvaluator.evaluate(GenericExpressionEvaluator.java:42) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.plugins.processor.mutateevent.AddEntryProcessor.doExecute(AddEntryProcessor.java:61) ~[mutate-event-processors-2.7.0.jar:?]
	at org.opensearch.dataprepper.model.processor.AbstractProcessor.lambda$execute$0(AbstractProcessor.java:54) ~[data-prepper-api-2.7.0.jar:?]
	at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:69) [micrometer-core-1.11.5.jar:1.11.5]
	at org.opensearch.dataprepper.model.processor.AbstractProcessor.execute(AbstractProcessor.java:54) [data-prepper-api-2.7.0.jar:?]
	at org.opensearch.dataprepper.pipeline.ProcessWorker.doRun(ProcessWorker.java:135) [data-prepper-core-2.7.0.jar:?]
	at org.opensearch.dataprepper.pipeline.ProcessWorker.run(ProcessWorker.java:61) [data-prepper-core-2.7.0.jar:?]
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539) [?:?]
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
	at java.base/java.lang.Thread.run(Thread.java:840) [?:?]
Caused by: org.opensearch.dataprepper.expression.ExpressionEvaluationException: Unable to evaluate the part of input statement: /my-list/-1
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:41) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:17) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.GenericExpressionEvaluator.evaluate(GenericExpressionEvaluator.java:39) ~[data-prepper-expression-2.7.0.jar:?]
	... 11 more
Caused by: org.opensearch.dataprepper.expression.ExpressionEvaluationException: Unable to evaluate the part of input statement: /my-list/-1
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.exitEveryRule(ParseTreeEvaluatorListener.java:91) ~[data-prepper-expression-2.7.0.jar:?]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.exitRule(ParseTreeWalker.java:63) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:38) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:37) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:17) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.GenericExpressionEvaluator.evaluate(GenericExpressionEvaluator.java:39) ~[data-prepper-expression-2.7.0.jar:?]
	... 11 more
Caused by: java.lang.IllegalArgumentException: DIVIDE requires left operand to be either Float or Integer.
	at org.opensearch.dataprepper.expression.ArithmeticBinaryOperator.evaluate(ArithmeticBinaryOperator.java:46) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ArithmeticBinaryOperator.evaluate(ArithmeticBinaryOperator.java:16) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.performSingleOperation(ParseTreeEvaluatorListener.java:104) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.exitEveryRule(ParseTreeEvaluatorListener.java:88) ~[data-prepper-expression-2.7.0.jar:?]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.exitRule(ParseTreeWalker.java:63) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:38) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:36) ~[antlr4-runtime-4.10.1.jar:4.10.1]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:37) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluator.evaluate(ParseTreeEvaluator.java:17) ~[data-prepper-expression-2.7.0.jar:?]
	at org.opensearch.dataprepper.expression.GenericExpressionEvaluator.evaluate(GenericExpressionEvaluator.java:39) ~[data-prepper-expression-2.7.0.jar:?]
	... 11 more

Start by tracing expression evaluation through AddEntryProcessor, GenericExpressionEvaluator, ParseTreeEvaluator, and ArithmeticBinaryOperator, using the reported /my-list/-1 failure as the entry point. Clarify the intended list-indexing behavior and edge cases, then verify that add_entries can retrieve the final list item and produce the requested lastkey value.