send backend metrics to their own postgres channel (#843)

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> [!IMPORTANT]
> Separate backend metrics into their own PostgreSQL channel using
`emit_backend_metrics` in `subscribe.rs`.
> 
>   - **Behavior**:
> - `publish_metrics` in `backend.rs` now uses `emit_backend_metrics` to
send metrics to `plane_backend_metrics` channel.
> - Removes `emit_ephemeral_with_key` and `emit_ephemeral_impl` from
`subscribe.rs`.
>   - **Functions**:
> - Adds `emit_backend_metrics` in `subscribe.rs` to handle backend
metrics separately.
>     - Modifies `emit_impl` to support new channel logic.
>   - **Constants**:
> - Adds `BACKEND_METRICS_EVENT_CHANNEL` in `subscribe.rs` for backend
metrics.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=jamsocket%2Fplane&utm_source=github&utm_medium=referral)<sup>
for b6ec34a. It will automatically
update as commits are pushed.</sup>

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plane/src/database/backend.rs

@@ -1,5 +1,5 @@
 use super::{
-    subscribe::{emit_ephemeral_with_key, emit_with_key},
+    subscribe::{emit_backend_metrics, emit_with_key},
     PlaneDatabase,
 };
 use crate::{
@@ -455,7 +455,7 @@ impl<'a> BackendDatabase<'a> {
 
     pub async fn publish_metrics(&self, metrics: BackendMetricsMessage) -> sqlx::Result<()> {
         let mut txn = self.db.pool.begin().await?;
-        emit_ephemeral_with_key(&mut txn, &metrics.backend_id.to_string(), &metrics).await?;
+        emit_backend_metrics(&mut txn, &metrics.backend_id.to_string(), &metrics).await?;
         txn.commit().await?;
         Ok(())
     }

plane/src/database/subscribe.rs

@@ -1,4 +1,4 @@
-use crate::database::util::MapSqlxError;
+use crate::database::{backend::BackendMetricsMessage, util::MapSqlxError};
 use crate::util::ExponentialBackoff;
 use chrono::{DateTime, Utc};
 use serde::{de::DeserializeOwned, Deserialize, Serialize};
@@ -17,7 +17,8 @@ use tokio::{
 
 type ListenerMap = Arc<RwLock<HashMap<(String, Option<String>), Box<dyn TypedSender>>>>;
 
-const EVENT_CHANNEL: &str = "plane_events";
+pub const EVENT_CHANNEL: &str = "plane_events";
+pub const BACKEND_METRICS_EVENT_CHANNEL: &str = "plane_backend_metrics";
 
 pub trait NotificationPayload:
     Serialize + DeserializeOwned + Debug + Send + Sync + Clone + 'static
@@ -382,12 +383,26 @@ pub async fn emit_impl<T: NotificationPayload>(
     Ok(())
 }
 
-pub async fn emit_ephemeral_impl<T: NotificationPayload>(
+pub async fn emit<T: NotificationPayload>(
     db: &mut PgConnection,
-    key: Option<&str>,
     payload: &T,
 ) -> Result<(), sqlx::Error> {
-    let kind = T::kind().to_string();
+    emit_impl(db, None, payload).await
+}
+
+pub async fn emit_with_key<T: NotificationPayload>(
+    db: &mut PgConnection,
+    key: &str,
+    payload: &T,
+) -> Result<(), sqlx::Error> {
+    emit_impl(db, Some(key), payload).await
+}
+
+pub async fn emit_backend_metrics(
+    db: &mut PgConnection,
+    key: &str,
+    payload: &BackendMetricsMessage,
+) -> Result<(), sqlx::Error> {
     sqlx::query!(
         r#"
         select pg_notify(
@@ -399,43 +414,13 @@ pub async fn emit_ephemeral_impl<T: NotificationPayload>(
                 'key', $2::text
             )::text
         )"#,
-        kind,
+        BackendMetricsMessage::kind().to_string(),
         key,
         serde_json::to_value(&payload).map_sqlx_error()?,
-        EVENT_CHANNEL,
+        BACKEND_METRICS_EVENT_CHANNEL,
     )
     .execute(db)
     .await?;
 
     Ok(())
 }
-
-pub async fn emit<T: NotificationPayload>(
-    db: &mut PgConnection,
-    payload: &T,
-) -> Result<(), sqlx::Error> {
-    emit_impl(db, None, payload).await
-}
-
-pub async fn emit_with_key<T: NotificationPayload>(
-    db: &mut PgConnection,
-    key: &str,
-    payload: &T,
-) -> Result<(), sqlx::Error> {
-    emit_impl(db, Some(key), payload).await
-}
-
-pub async fn emit_ephemeral<T: NotificationPayload>(
-    db: &mut PgConnection,
-    payload: &T,
-) -> Result<(), sqlx::Error> {
-    emit_ephemeral_impl(db, None, payload).await
-}
-
-pub async fn emit_ephemeral_with_key<T: NotificationPayload>(
-    db: &mut PgConnection,
-    key: &str,
-    payload: &T,
-) -> Result<(), sqlx::Error> {
-    emit_ephemeral_impl(db, Some(key), payload).await
-}