Add Prometheus metrics.

Cargo.lock

@@ -1971,6 +1971,20 @@ dependencies = [
  "yansi",
 ]
 
+[[package]]
+name = "prometheus"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c8425533e7122f0c3cc7a37e6244b16ad3a2cc32ae7ac6276e2a75da0d9c200d"
+dependencies = [
+ "cfg-if 1.0.0",
+ "fnv",
+ "lazy_static",
+ "parking_lot 0.11.1",
+ "regex",
+ "thiserror",
+]
+
 [[package]]
 name = "quick-error"
 version = "1.2.3"
@@ -2351,7 +2365,7 @@ dependencies = [
 
 [[package]]
 name = "revolt"
-version = "0.3.2"
+version = "0.3.3"
 dependencies = [
  "async-std",
  "async-tungstenite",
@@ -2377,6 +2391,7 @@ dependencies = [
  "rocket",
  "rocket_contrib",
  "rocket_cors",
+ "rocket_prometheus",
  "serde",
  "serde_json",
  "snafu",
@@ -2493,6 +2508,15 @@ dependencies = [
  "version_check",
 ]
 
+[[package]]
+name = "rocket_prometheus"
+version = "0.7.0"
+source = "git+https://github.com/insertish/rocket_prometheus?rev=3d825aedb42793246c306a81fe67c5b187948983#3d825aedb42793246c306a81fe67c5b187948983"
+dependencies = [
+ "prometheus",
+ "rocket",
+]
+
 [[package]]
 name = "rust-argon2"
 version = "0.8.3"