Q:

Origin of "off the meter" idiomatic phrase

When and how did the phrase "off the meter" become established as an idiom?
Urban Dictionary defines "off the meter" as the condition of being "very good, awesome, great". 
I have heard and said it on impulse to mean the great extent.

A:

I can think of three possible senses of "off the meter" that might account for the current popular phrase: (1) out of bounds, beyond the dial readings of a light, sound, or other type of gauge, as when the indicator arrow on a meter seems stuck at its extreme high end; (2) unregulated, as when a cab driver turns of the car's fare meter and drives for a special price or for free; (3) syncopated, so that the emphasis of the rhythm falls elsewhere than on the expected beat.
The meaning cited as possible source #1 appears in Frank Herbert, The Dragon in the Sea (1956) [compiled from snippets]:

Sparrow opened the exchange valve. Blood from Garcia's body began to flow into the unit's lead-lined storage system as the new blood was pumped into his body. Immediately, Ramsey's monitor snooper swung far right, stuck there.
"He's off the meter, skipper."
...
Ramsey fell silent, watching the monitor dial. It stayed against the right-hand pin. "I got his shots into him and took my own before you came up," said Sparrow. "We'd better check you now."
"Go ahead," said Ramsey. He held out his left arm, kept his gaze on the monitor dial. "Three changes through him by now for sure and he's still off the meter."

The meaning cited as possible source #2 appears in Jerry Oster, Sweet Justice (1985), reissued a year later as Rough Justice [compiled from snippets]:

"I asked him and he said he thought maybe, he wasn't sure, but maybe the cab was off duty — you know, had his off-duty lights on. If that's so, loo, and the cabby was heading home, he might not've logged the trip, just did it, you know, off the meter and pocketed the fare himself. When Grace and I were, you know, having that trouble, and I was moonlighting driving a cab to pay the extra rent 'cause I had my own apartment for a while along with the house, I used to do that now and then, you know, on my last trip of the night."

The meaning cited as possible source #3 appears in Michael Tenzer, Gamelan Gong Kebyar: The Art of Twentieth-Century Balinese Music (2000):

The vocabulary of stock ocak-ocakan rhythms is related to angsel rhythms, in that they play off the meter with insistent syncopation and, in dance music, are directly tied to detailed movements. Composers also invent new ones as frequently as they reuse the old.

One of the first metaphorical use of "off the meter" that a Google Books search finds is from Sean Henry, "Comic Threat," in Mother Jones magazine (November/December 1994):

It's difficult to blame them. Diana isn't the boy next door; his artistic tastes, when compared to the mainstream, are completely off the meter. Whether it's death or excrement, or simply shapes that make no sense, most of Diana's material leaves viewers wondering, "What's wrong with this kid?"

I think that this instance of "off the meter" probably derives from possible source #1 above, and I suspect that current usage derives from the same source.
Consistent with terpy's answer, the first instance of "off the meter" as a synonym for "very good, awesome, great" to appear in a Google Books search is this one, from a caption in "The Vibe Spot" in Vibe magazine (August 1998):

Throw your hands in the air if you love Enyce, Alizé, and hip hop! The Fat Black Pussycat kickoff party at the hotter-than-July Miami nightspot Liquid was off the meter!

