Bring back legacy proto

Signed-off-by: John Howard <john.howard@solo.io>

Author: howardjohn

CHANGELOG.md

@@ -18,6 +18,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `io.prometheus.client` protobuf messages from `metrics.proto` rather than the
   OpenMetrics protobuf data model. This is a breaking change for users of the `protobuf` feature.
   See [Issue](https://github.com/prometheus/OpenMetrics/issues/296) for more context.
+  The `legacy_protobuf` feature retains the old, deprecated, OpenMetrics protobuf support which will
+  be removed in a future release.
 
 ## [0.24.1]
 

Cargo.toml

@@ -13,6 +13,7 @@ documentation = "https://docs.rs/prometheus-client"
 [features]
 default = []
 protobuf = ["dep:prost", "dep:prost-types", "dep:prost-build"]
+legacy_protobuf = ["dep:prost", "dep:prost-types", "dep:prost-build"]
 protobuf-protox = ["protobuf", "dep:protox"]
 
 # This feature provides additional APIs for testing downstream code using

build.rs

@@ -1,19 +1,26 @@
 use std::error::Error;
 
 fn main() -> Result<(), Box<dyn Error>> {
-    #[cfg(feature = "protobuf")]
+    #[cfg(any(feature = "protobuf", feature = "legacy_protobuf"))]
     compile_protos()?;
 
     Ok(())
 }
 
-#[cfg(feature = "protobuf")]
+#[allow(clippy::vec_init_then_push)] // False positive due to feature flags
+#[cfg(any(feature = "protobuf", feature = "legacy_protobuf"))]
 fn compile_protos() -> Result<(), Box<dyn Error>> {
-    let protos = ["src/encoding/proto/metrics.proto"];
+    let mut protos = Vec::new();
+
+    #[cfg(feature = "protobuf")]
+    protos.push("src/encoding/proto/metrics.proto");
+    #[cfg(feature = "legacy_protobuf")]
+    protos.push("src/encoding/proto/openmetrics_data_model.proto");
+
     let includes = ["src/encoding/proto/"];
 
     #[cfg(feature = "protobuf-protox")]
-    prost_build::compile_fds(protox::compile(protos, includes)?)?;
+    prost_build::compile_fds(protox::compile(&protos, includes)?)?;
 
     #[cfg(not(feature = "protobuf-protox"))]
     prost_build::compile_protos(&protos, &includes)?;