barman-exporter not compatible with prometheus-client 0.15.0

[Robbie558]

Installation of the barman-exporter package via pip3 resolves the prometheus-client dependency and install version 0.15.0, which causes the following failure when you run / start the service:

$ /usr/local/bin/barman-exporter
Traceback (most recent call last):
  File "/usr/local/bin/barman-exporter", line 7, in <module>
    from barman_exporter.barman_exporter import main
  File "/usr/local/lib/python3.5/dist-packages/barman_exporter/barman_exporter.py", line 8, in <module>
    import prometheus_client
  File "/usr/local/lib/python3.5/dist-packages/prometheus_client/__init__.py", line 3, in <module>
    from . import (
  File "/usr/local/lib/python3.5/dist-packages/prometheus_client/exposition.py", line 194
    timestamp = f' {int(float(line.timestamp) * 1000):d}'

I had to downgrade to prometheus-client 0.9.0 to clear.