Who is the herald mentioned in the noble prophetic traditions with the saying: "And a herald called out"?

The phrase "a caller called out to me" or "a caller called out" appears in the hadiths with different meanings. The caller might be Allah, the Exalted and Glorified, as in the Night Journey when He called out: "I have ratified My ordinance and lightened the burden for My servants." Or the caller might be an angel, as in the hadith: "And a caller calls out every night: 'O seeker of good, come forth!'" or in the saying: "A caller called out to him: 'You are good, and your walk is good, and you have taken your abode in Paradise.'"