Fix custom metric display (apache#20643)

## Which issue does this PR close?

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- Closes part of
datafusion-contrib/datafusion-distributed#361.

## Rationale for this change

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The custom metrics display is unnecessarily prepending the name of the
metric during the display:

```
network_latency_min_2=name:network_latency_min 274.88µs
```

When it should just be:
```
network_latency_min_2=274.88µs
```

## What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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Fix to the display

## Are these changes tested?

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yes, by a new test

## Are there any user-facing changes?

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(cherry picked from commit 657887d)

Author: gabotechs

datafusion/physical-expr-common/src/metrics/value.rs

@@ -1050,8 +1050,8 @@ impl Display for MetricValue {
                 write!(f, "{pruning_metrics}")
             }
             Self::Ratio { ratio_metrics, .. } => write!(f, "{ratio_metrics}"),
-            Self::Custom { name, value } => {
-                write!(f, "name:{name} {value}")
+            Self::Custom { value, .. } => {
+                write!(f, "{value}")
             }
         }
     }
@@ -1146,6 +1146,12 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_display_custom_metric() {
+        let custom_val = new_custom_counter("hi", 11);
+        assert_eq!(custom_val.to_string(), "count: 11");
+    }
+
     #[test]
     fn test_display_output_rows() {
         let count = Count::new();