Enhance convertGraphiteTargetToPromQL()

[rodio]

• In the previous version of the convertGraphiteTargetToPromQL function, an asterisk was matched to one or more symbols. Still, according to the graphite documentation it should match zero or more characters, so this is fixed in this PR.
• It did not escape all the possible regexp symbols that can be part of the query. This is fixed by regexp.QuoteMeta() now.
• For some reason, four backslashes were used to escape dots. We did not understand why so this new version adds two backslashes for each dot: one is added by regexp.QuoteMeta() and another by %q formatting instead of %s.

Thanks, @valyala for the convertGraphiteTargetToPromQL function.