Merge pull request #2 from ackintosh/protobuf-support-associated-type

Make `encode` return iterator instead of vec

Author: ackintosh

Cargo.toml

@@ -24,6 +24,7 @@ prometheus-client-derive-proto-encode = { version = "0.1.0", path = "derive-prot
 prometheus-client-derive-text-encode = { version = "0.3.0", path = "derive-text-encode" }
 prost = { version = "0.9.0", optional = true }
 prost-types = { version = "0.9.0", optional = true }
+void = "1.0"
 
 [dev-dependencies]
 async-std = { version = "1", features = ["attributes"] }