feat: add promslog.NewNopLogger() convenience func

Simple convenience function to return an slog.Logger that writes to
io.Discard. Originally suggested by @ArthurSens
[here](prometheus#686 (comment)),
and requested again by @bboreham
[here](prometheus/prometheus#14906 (comment)).
As Bryan points out in the comment, there's 147 instances where a
discard logger is needed, so a consistent utility function to manage
them seems helpful.

Signed-off-by: TJ Hoplock <t.hoplock@gmail.com>

promslog/slog.go

@@ -180,3 +180,9 @@ func New(config *Config) *slog.Logger {
 	}
 	return slog.New(slog.NewTextHandler(config.Writer, logHandlerOpts))
 }
+
+// NewNopLogger is a convenience function to return an slog.Logger that writes
+// to io.Discard.
+func NewNopLogger() *slog.Logger {
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}