examples: Update to metrics-exporter-prometheus 0.18 (#3590)

Author: tottoto

examples/Cargo.lock

@@ -7,7 +7,7 @@ publish = false
 [dependencies]
 axum = { path = "../../axum" }
 metrics = { version = "0.24", default-features = false }
-metrics-exporter-prometheus = { version = "0.17", default-features = false }
+metrics-exporter-prometheus = { version = "0.18", default-features = false }
 tokio = { version = "1.0", features = ["full"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }