The PR removes code related to #202
Basically, since all instances now have hostname attached, a simple proxy should be able to solve the issue
I took some time to implement custom labeling, but it required way more logic than it gives benefits:

• config had to have custom logic for:
  a. verifying label consistency
  b. sorting label keys
  c. sorting label values according to keys order
• client had to preserve either reference to config or reference to sorted slice of keys that are later on used to determine values to add to each and every metric
• since Allow to specify multiple logstash instances #79 was closed, there is no simple way that i know to treat these labels as const labels. therefore the logic is mixed with existing variableLabels logic which makes code way more unreadable than it currently is

I believe there are existing solutions allowing to relabel prometheus metrics.
In most cases hostname label alone should be sufficient, but if you are running multiple instances monitoring for example a local url you may consider using combination of both exporter and logstash address as a base for relabeling