Fix HTTP header for the remote write exporting connector

[vlvkobal]

Summary

Fixes #7676

Component Name

exporting engine, backends

Test Plan

1. Configure an exporting connector

[prometheus_remote_write:test_instance]
enabled = yes
destination = localhost:1234
remote write URL path = /receive
data source = as collected
update every = 2
send charts matching = system.processes

Check the HTTP Host filed using nc -l -p 1234, it should be set to localhost:1234.

2. Configure the backends subsytem

[backend]
  enabled = yes
  type = prometheus_remote_write
  data source = as collected
  destination = localhost:1234
  send charts matching = system.processes

Check the HTTP Host filed using nc -l -p 1234, it should be set to localhost:1234.

[thiagoftsm]

This PR is working as expected and it is fixing the issue, but I would like to call attention that we are reading the destination value and we are not parsing it inside read_config.c to test if it is a valid value, but this is subject for another possible PR.
LGTM!