docs: add example using std::io::Write (#261)

Signed-off-by: Edwin Amsler <[email protected]>

Author: RandomInsano

examples/io_write.rs

@@ -0,0 +1,39 @@
+//! Example showing how one could write to a file or socket instead of a string.
+//! For large metrics registries this will be more memory efficient.
+
+use prometheus_client::{encoding::text::encode, metrics::counter::Counter, registry::Registry};
+use std::io::Write;
+
+fn main() {
+    let mut registry = <Registry>::with_prefix("stream");
+    let request_counter: Counter<u64> = Default::default();
+
+    registry.register(
+        "requests",
+        "How many requests the application has received",
+        request_counter.clone(),
+    );
+
+    let mut buf = String::new();
+    encode(&mut buf, &registry).unwrap();
+
+    let mut file = Vec::new();
+    let mut writer = IoWriterWrapper(&mut file);
+    encode(&mut writer, &registry).unwrap();
+
+    assert!(buf.as_bytes() == file);
+}
+
+pub struct IoWriterWrapper<W>(W);
+
+impl<W> std::fmt::Write for IoWriterWrapper<W>
+where
+    W: Write,
+{
+    fn write_str(&mut self, input: &str) -> std::fmt::Result {
+        self.0
+            .write_all(input.as_bytes())
+            .map(|_| ())
+            .map_err(|_| std::fmt::Error)
+    }
+}