Added command polars len for performing count(*) like operations. (#13941)

# Description
This request exposes the prelude::polars::len expression. It is ended
for doing fast select count(*) like operations:

<img width="626" alt="Screenshot 2024-09-26 at 18 14 45"
src="https://github.com/user-attachments/assets/74285fc6-f99c-46e0-9226-9a7d41738d78">

# User-Facing Changes
- Introduction of the `polars len` command

crates/nu_plugin_polars/src/dataframe/command/data/len.rs

@@ -0,0 +1,76 @@
+use crate::{
+    values::{Column, CustomValueSupport, NuDataFrame, NuExpression},
+    PolarsPlugin,
+};
+
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{Category, Example, LabeledError, PipelineData, Signature, Span, Type, Value};
+
+#[derive(Clone)]
+pub struct ExprLen;
+
+impl PluginCommand for ExprLen {
+    type Plugin = PolarsPlugin;
+
+    fn name(&self) -> &str {
+        "polars len"
+    }
+
+    fn description(&self) -> &str {
+        "Return the number of rows in the context. This is similar to COUNT(*) in SQL."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_type(Type::Any, Type::Custom("expression".into()))
+            .category(Category::Custom("dataframe".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Count the number of rows in the the dataframe.",
+                example: "[[a b]; [1 2] [3 4]] | polars into-df | polars select (polars len) | polars collect",
+                result: Some(
+                    NuDataFrame::try_from_columns(
+                        vec![
+                            Column::new("len".to_string(), vec![Value::test_int(2)]),
+                        ],
+                        None,
+                    )
+                    .expect("simple df for test should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
+            Example {
+                description: "Creates a last expression from a column",
+                example: "polars col a | polars last",
+                result: None,
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        plugin: &Self::Plugin,
+        engine: &EngineInterface,
+        call: &EvaluatedCall,
+        _input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let res: NuExpression = polars::prelude::len().into();
+        res.to_pipeline_data(plugin, engine, call.head)
+            .map_err(LabeledError::from)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::test::test_polars_plugin_command;
+    use nu_protocol::ShellError;
+
+    #[test]
+    fn test_examples() -> Result<(), ShellError> {
+        test_polars_plugin_command(&ExprLen)
+    }
+}

crates/nu_plugin_polars/src/dataframe/command/data/mod.rs

@@ -19,6 +19,7 @@ mod flatten;
 mod get;
 mod join;
 mod last;
+mod len;
 mod lit;
 mod pivot;
 mod query_df;
@@ -85,6 +86,7 @@ pub(crate) fn data_commands() -> Vec<Box<dyn PluginCommand<Plugin = PolarsPlugin
         Box::new(UnpivotDF),
         Box::new(FirstDF),
         Box::new(LastDF),
+        Box::new(len::ExprLen),
         Box::new(RenameDF),
         Box::new(SampleDF),
         Box::new(SliceDF),