Resolve "Add support for more features in query builder API"

Closes #39 (closed)

src/query/ops/modifiers.rs

 use std::fmt::Display;
 
+use crate::{query::IntoQuery, seal::Sealed};
+
+use super::Operable;
+
 #[derive(Debug, Clone, Default)]
 pub(crate) struct Mod<'a, M> {
     mod_type: M,
@@ -114,3 +118,42 @@ impl Display for Mod<'_, AggrType> {
         Ok(())
     }
 }
+
+/// MetricsQL's `keep_metric_names` modifier
+///
+/// This modifier can only be applied using the `keep_metric_names` method made available by the [`QueryExt`](crate::query::QueryExt) trait.
+#[cfg(feature = "metricsql")]
+#[derive(Debug, Clone)]
+pub struct KeepNames<E: Operable>(pub(crate) E);
+
+#[cfg(feature = "metricsql")]
+impl<E: Operable> Display for KeepNames<E> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{exp} keep_metric_names", exp = self.0)
+    }
+}
+
+#[cfg(feature = "metricsql")]
+impl<E: Operable> Sealed for KeepNames<E> {}
+#[cfg(feature = "metricsql")]
+impl<E: Operable> Operable for KeepNames<E> {}
+
+#[cfg(feature = "metricsql")]
+impl<E: Operable> IntoQuery for KeepNames<E> {
+    type Target = String;
+    fn into_query(self) -> Self::Target {
+        self.to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::query::{Metric, QueryExt};
+
+    #[cfg(feature = "metricsql")]
+    #[test]
+    fn keep_names() {
+        let qry = Metric::new("metric").keep_metric_names();
+        assert_eq!(qry.to_string(), "metric keep_metric_names");
+    }
+}