add prometheus metrics

Cargo.toml

@@ -8,7 +8,7 @@ debug = 1
 
 [dependencies]
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
-tokio = { version = "1.52", features = ["full"] }
+tokio = { version = "1.53", features = ["full"] }
 tokio-stream = "0.1"
 tokio-util = "0.7"
 futures-util = "0.3"
@@ -16,7 +16,7 @@ serde_json = "1.0"
 serde = { version = "1.0", features = ["derive"] }
 sentry = { version = "0.48", features = ["tower-axum-matched-path"] }
 sentry-anyhow = { version = "0.48", features = ["backtrace"] }
-openrouter-rs = "0.10"
+openrouter-rs = "0.12"
 dotenvy = "0.15"
 tower = "0.5"
 tower-http = {version = "0.6", features = ["trace"] }
@@ -28,3 +28,5 @@ thiserror = "2.0"
 ring = "0.17"
 hex = "0.4"
 bytes = "1.1"
+metrics = "0.24"
+metrics-exporter-prometheus = { version = "0.18", default-features = false, features = ["http-listener"] }