Export volume metrics

[umatare5]

Hi @marcopaganini

This PR allows to export the volume.

• For further information, please refer to https://github.com/marcopaganini/quotes-exporter/issues/1.

Summary

• Implement --quote.volume flag.

$ ./quotes-exporter --help
Usage of ./quotes-exporter:
  -port int
        Port to listen for HTTP requests. (default 9340)
  -quote.volume
        Exports volume.

• Returns quotes_exporter_volume when enabled --quote.volume.

& curl -sS 'http://localhost:9340/price?symbols=AMZN,GOOG,SNAP' | grep quotes_exporter_volume
# HELP quotes_exporter_volume Asset Volume.
# TYPE quotes_exporter_volume gauge
quotes_exporter_volume{name="Alphabet Inc.",symbol="GOOG"} 1.648353e+06
quotes_exporter_volume{name="Amazon.com, Inc.",symbol="AMZN"} 5.7799e+06
quotes_exporter_volume{name="Snap Inc.",symbol="SNAP"} 2.1831525e+07

[umatare5]

Thank you for your reviewing.

I fixed them. Please check it when you have a time.

[marcopaganini]

Merged. Thanks for the contribution!

[umatare5]

Likewise!