fix(hermes): fix metrics content-type

Openmetrics specification expects its own content-type instead of plain
text. I didn't set it before as Prometheus still accepts text format but
apparently there are some scrapers (like datadog) that decide
dynamically what format to use for parsing based on the content type and
our format follows Openmetrics specification. Prometheus should be able
to parse it but weirdly it didn't work in our situation. An example of
inconsistency is how counters are represented. In Openmetrics
specification the metric name is appended by `_total` whereas normal
prometheus just uses the metric name.

hermes/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name        = "hermes"
-version     = "0.4.1"
+version     = "0.4.2"
 description = "Hermes is an agent that provides Verified Prices from the Pythnet Pyth Oracle."
 edition     = "2021"
 

hermes/src/metrics_server.rs

@@ -10,6 +10,7 @@ use {
     anyhow::Result,
     axum::{
         extract::State,
+        http::header,
         response::IntoResponse,
         routing::get,
         Router,
@@ -55,5 +56,11 @@ pub async fn metrics(State(state): State<Arc<AppState>>) -> impl IntoResponse {
     // to write to the buffer.
     encode(&mut buffer, &registry).unwrap();
 
-    buffer
+    (
+        [(
+            header::CONTENT_TYPE,
+            "application/openmetrics-text; version=1.0.0; charset=utf-8",
+        )],
+        buffer,
+    )
 }