fix(metrics-exporter-prometheus): fix broken native histograms serialization (#649)

Author: adelsz

metrics-exporter-prometheus/Cargo.toml

@@ -77,7 +77,7 @@ prost-build = { workspace = true, optional = true }
 
 [[example]]
 name = "native_histograms"
-required-features = ["http-listener"]
+required-features = ["http-listener", "protobuf"]
 
 [[example]]
 name = "prometheus_push_gateway"

metrics-exporter-prometheus/src/protobuf.rs

@@ -164,6 +164,7 @@ pub(crate) fn render_protobuf(
                     }
                 }
                 Distribution::NativeHistogram(native_hist) => {
+                    metric_type = Some(pb::MetricType::Histogram);
                     // Convert our native histogram into Prometheus native histogram format
                     let positive_buckets = native_hist.positive_buckets();
                     let negative_buckets = native_hist.negative_buckets();