count federation requests by server name

This resuls in very high cardinality, so probably not something to run
for a long period of time.

src/api/server_server.rs

@@ -12,6 +12,7 @@ use std::{
 use axum::{response::IntoResponse, Json};
 use axum_extra::headers::{Authorization, HeaderMapExt};
 use get_profile_information::v1::ProfileField;
+use opentelemetry::KeyValue;
 use ruma::{
     api::{
         client::error::{Error as RumaError, ErrorKind},
@@ -703,6 +704,10 @@ pub(crate) async fn send_transaction_message_route(
     let sender_servername =
         body.sender_servername.as_ref().expect("server is authenticated");
 
+    METRICS
+        .federation_requests
+        .add(1, &[KeyValue::new("server_name", sender_servername.to_string())]);
+
     let mut resolved_map = BTreeMap::new();
 
     let pub_key_map = RwLock::new(BTreeMap::new());

src/observability.rs

@@ -155,6 +155,9 @@ pub(crate) struct Metrics {
 
     /// Counts where data is found from
     lookup: opentelemetry::metrics::Counter<u64>,
+
+    /// Counts incoming federation requests by origin
+    pub(crate) federation_requests: opentelemetry::metrics::Counter<u64>,
 }
 
 impl Metrics {
@@ -205,10 +208,16 @@ impl Metrics {
             .with_description("Counts where data is found from")
             .init();
 
+        let federation_requests = meter
+            .u64_counter("federation_requests")
+            .with_description("Counts incoming federation requests by origin")
+            .init();
+
         Metrics {
             otel_state: (registry, provider),
             http_requests_histogram,
             lookup,
+            federation_requests,
         }
     }