minor: run all examples by default (#19506)

## Which issue does this PR close?

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- Closes #.

## Rationale for this change

A small fix-up to run
```sh
cargo run --example relation_planner
```
instead of
```sh
cargo run --example relation_planner -- all
```

If no arguments are specified, run all examples. It conforms to the
expected behaviour of cargo examples.

Extracted from to
#19311 (comment)

## What changes are included in this PR?


## Are these changes tested?

Manual tests.

## Are there any user-facing changes?

No

Author: theirix

datafusion-examples/examples/builtin_functions/main.rs

@@ -80,7 +80,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/custom_data_source/main.rs

@@ -108,7 +108,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/data_io/main.rs

@@ -116,7 +116,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/dataframe/main.rs

@@ -85,7 +85,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/execution_monitoring/main.rs

@@ -84,7 +84,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/external_dependency/main.rs

@@ -76,7 +76,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/flight/main.rs

@@ -87,7 +87,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/proto/main.rs

@@ -74,7 +74,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/query_planning/main.rs

@@ -100,7 +100,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;
 

datafusion-examples/examples/relation_planner/main.rs

@@ -90,7 +90,7 @@ async fn main() -> Result<()> {
 
     let example: ExampleKind = std::env::args()
         .nth(1)
-        .ok_or_else(|| DataFusionError::Execution(format!("Missing argument. {usage}")))?
+        .unwrap_or_else(|| ExampleKind::All.to_string())
         .parse()
         .map_err(|_| DataFusionError::Execution(format!("Unknown example. {usage}")))?;