Fix: replace invalid metric tag values with an empty string instead of _

[markushi]

According to our spec invalid tag values should be replaced with no string.

Tag Values: regex [^\w\d_:/@\.{}\[\]$-]+ with no replacement character.

I cross checked with PHP and JS: both of them also use no replacement character.

Relevant Python PR: getsentry/sentry-python#2773

[jamescrosswell]

Thanks @markushi !

[bruno-garcia]

this is higher chance to cause conflict no?

abc&&a
abc*a
abc::::a

all become abca?

[stefanosiano]

not really, as it became abc_a previously for all 3 cases. Also, this is for tag values only, not keys, so we should be fine