fix(deps): update module github.com/prometheus/client_golang to v1.21.0

Author: larsl-renovate[bot]

go.mod

@@ -12,7 +12,7 @@ require (
 	github.com/henrywhitaker3/flow v1.10.0
 	github.com/labstack/echo-contrib v0.17.2
 	github.com/labstack/echo/v4 v4.13.3
-	github.com/prometheus/client_golang v1.20.5
+	github.com/prometheus/client_golang v1.21.0
 	github.com/prometheus/common v0.62.0
 	github.com/spf13/cobra v1.8.1
 	github.com/spf13/pflag v1.0.6