fix: harden high-fanout delivery under load

server/db/mysql/metrics_driver.go

@@ -113,6 +113,12 @@ func (c instrumentedConn) QueryContext(ctx context.Context, query string, args [
 	started := time.Now()
 	rows, err := queryer.QueryContext(ctx, query, args)
 	immetrics.ObserveDB("mysql", "query", started, err)
+	if isInvalidConnection(err) {
+		// Queries are idempotent. Returning ErrBadConn makes database/sql discard
+		// this connection and transparently retry on a fresh one.
+		immetrics.IncDBInvalidConnectionRetry("query", "discard")
+		return nil, driver.ErrBadConn
+	}
 	return rows, err
 }
 
@@ -211,6 +217,10 @@ func (s instrumentedStmt) QueryContext(ctx context.Context, args []driver.NamedV
 	started := time.Now()
 	rows, err := queryer.QueryContext(ctx, args)
 	immetrics.ObserveDB("mysql", "query", started, err)
+	if isInvalidConnection(err) {
+		immetrics.IncDBInvalidConnectionRetry("prepared_query", "discard")
+		return nil, driver.ErrBadConn
+	}
 	return rows, err
 }