fix: unable to start pika exporter (OpenAtomFoundation#2640)

* delete defalut value of codisadr

* add description for using codisaddr

* Update main.go

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Co-authored-by: xiaodi05 <xiaodi05@meituan.com>
Co-authored-by: Xin.Zh <dragoncharlie@foxmail.com>

tools/pika_exporter/main.go

@@ -19,7 +19,7 @@ import (
 var (
 	hostFile           = flag.String("pika.host-file", getEnv("PIKA_HOST_FILE", ""), "Path to file containing one or more pika nodes, separated by newline. NOTE: mutually exclusive with pika.addr.")
 	addr               = flag.String("pika.addr", getEnv("PIKA_ADDR", ""), "Address of one or more pika nodes, separated by comma.")
-	codisaddr          = flag.String("codis.addr", getEnv("CODIS_ADDR", "http://localhost:port/topom"), "Address of one or more codis topom urls, separated by comma.")
+	codisaddr          = flag.String("codis.addr", getEnv("CODIS_ADDR", ""), "Address of one or more codis topom urls, separated by comma, such as \"http://localhost:port/topom\".")
 	password           = flag.String("pika.password", getEnv("PIKA_PASSWORD", ""), "Password for one or more pika nodes, separated by comma.")
 	alias              = flag.String("pika.alias", getEnv("PIKA_ALIAS", ""), "Pika instance alias for one or more pika nodes, separated by comma.")
 	namespace          = flag.String("namespace", getEnv("PIKA_EXPORTER_NAMESPACE", "pika"), "Namespace for metrics.")