I never shot a concert (and don't intend to), but being a musician myself, be aware that musicians and audience are very sensitive
to noise. This is particularly true for classical music and jazz. Therefore, use a Leica or a Rolleiflex (or something in the same league
decibel-wise) if you go to such a concert. Don't use a Hasselblad! even 35mm SLR are borderline in my opinion.
Obviously, this is irrelevant for hard rock concerts.
My worst ever related experience: I attended a classical concert which my then-girlfriend (a professional violinist) played.
A few rows behind me was a guy with a big, modern, professional-grade Canikon DSLR on a tripod. For what I know he might have been
a photog for the local press or so. He definitely looked like it. Believe me or not, he took one picture every few seconds
for the duration of the concert. The wonders of digital. I could have killed the guy. Needless to say it ruined the experience
for everyone sitting within 10 meters. That day I swore I would never shoot a concert but rather shoot insensitive photographers
By the way, many classical orchestras I played with did not allow photographs being taken during the concert. When they needed pics
e.g. for their website, they would stage a concert for this purpose. The photographer was also free to move around "inside"
the orchestra (try that in a real concert!) Better pics and no frustration to anyone.
To answer your question about metering:
Jim nailed it. Follow his advice. The light generally does not change, so measure once and you're good for the concert - unless of course
there is a big change in lighting conditions.