What does this PR do?

Fixes elastic/beats#33125

Because this problem makes me a tad paranoid, I decided to implement the fix on a more global level. Specifically, this changes the behavior of the NewX() monitoring functions so they don't panic when a variable of the same name and type is re-registered. This fix is a tad opinionated, and I'd like to hear input from others on a few things:

• Instead of panicking when a variable is re-registered, we return the existing variable. My concern is that this is a tad implicit, and code might be assuming that a call to NewX() will create a new monitoring variable with a zero value. Should we zero out the underlying metric, and then return?
• If someone tries to re-register a variable with options, we just revert to a panic. I figured this was the most understandable behavior, as certain options, such as expvar exporting, can't really be modified. Should we attempt to merge or reset variables where possible?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md