Need to install wood framing in a pipe filled wall to be able to hang shelving
So, you've got lots of options, some of which no "real" contractor would ever do, but are perfectly fine.

If you use long drywall screws you can screw your wood blocking to the metal studs, just like you would if the studs were wood.  Sharp screws will penetrate the metal studs.  This is not optimal because your skill at doing this non-standard solution could result in shelf failure later, though it's probably what I would do.

You could do as one of commenters suggested and put stand-alone shelving agains the wall and attach it with brackets to the new drywall or directly into the metal studs- perfectly fine - very DIYish.  Use drywall anchors.

You could screw shelves directly into the metal studs - they are studs - they hold weight - no problem.  This is also something I would probably do.  Use drywall anchors.

You could use 1/2 or 3/4 plywood on the wall instead of drywall.  Kind of odd, but it will provide excellent holding capacity to attach shelving. (I have a plywood wall in my mudroom - shelving bolted right on.

You could attach vertical wood 2x4x on top of the drywall floor to ceiling, attached to the studs (essentially a new set of exposed studs).  Then attach your shelving to that.

Basically, your goal is to get something that isn't going to easily rip away from the wall...and a nearly infinite variety of screws, wood and brackets will satisfy this.