Auto merge of #3959 - Turbo87:prom, r=pietroalbini

prometheus: Disable `protobuf` feature

see tikv/rust-prometheus#216

Author: bors

Cargo.lock

@@ -68,7 +68,7 @@ lettre = { version = "0.10.0-beta.3", default-features = false, features = ["fil
 license-exprs = "1.6"
 oauth2 = { version = "4.0.0", default-features = false, features = ["reqwest"] }
 parking_lot = "0.11"
-prometheus = "0.12.0"
+prometheus = { version = "0.12.0", default-features = false }
 rand = "0.8"
 reqwest = { version = "0.11", features = ["blocking", "gzip", "json"] }
 scheduled-thread-pool = "0.2.0"