feat(metrics): add Prometheus metrics and scrape endpoint

Add a prometheus metrics module with counters for requests, DDoS/scanner/
rate-limit decisions, active connections gauge, and request duration
histogram. Spawn a lightweight HTTP server on a configurable port
(default 9090) serving /metrics and /health endpoints.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

Cargo.lock

@@ -1796,9 +1796,9 @@ checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
 [[package]]
 name = "libc"
-version = "0.2.182"
+version = "0.2.183"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6800badb6cb2082ffd7b6a67e6125bb39f18782f793520caee8cb8846be06112"
+checksum = "b5b646652bf6661599e1da8901b3b9522896f01e736bad5f723fe7a3a27f899d"
 
 [[package]]
 name = "libz-ng-sys"
@@ -3449,6 +3449,7 @@ dependencies = [
  "pingora-core",
  "pingora-http",
  "pingora-proxy",
+ "prometheus",
  "rustc-hash",
  "rustls",
  "serde",