I taste a liquor never brewed

"Not all the Vats upon the Rhine Yield such an Alcohol!"

The speaker seems to be in ecstasy, and so enthusiastic about drinking.  Well, the speaker isn't actually drinking.  This is revealed quite plainly in the title and  in the first line.  So, then the alcohol that he never actually brewed is a metaphor for, to be cliche, being high on life.  The speaker does not seem inebriated off any real substance, but does seem to be happy to be alive.  Another metaphor I noticed is in the last stanza, "leaning against the-Sun-."  The speaker cannot conceivably be leaning against the sun, as he would be incinerated in a fraction of a second.  The sun to me is the source of light.  Light equates to brightness.  Brightness is always associated with optimism and feelings of joy.  So, the speaker, so close to the sun, is evidently enjoying himself, and not caring one bit about anything else ("I shall but drink the more!").