Move to github.com/prometheus/common/promlog for logging

[johanfleury]

This PR change logging framework from github.com/prometheus/common/log to github.com/prometheus/common/promlog as the former was removed in v0.27.0.

This changes quite a few things, especially the log format:

$ go run ssl_exporter.go
INFO[0000] Starting ssl_exporter (version=, branch=, revision=)  source="ssl_exporter.go:130"
INFO[0000] Build context (go=go1.15.13, user=, date=)    source="ssl_exporter.go:131"
INFO[0000] Listening on :9219                            source="ssl_exporter.go:148"
^Csignal: interrupt

$ go run ssl_exporter.go
ts=2021-06-10T18:20:51.579Z caller=log.go:168 level=info msg="Starting ssl_exporter (version=, branch=, revision=)"
ts=2021-06-10T18:20:51.579Z caller=log.go:168 level=info msg="Build context (go=go1.15.13, user=, date=)"
ts=2021-06-10T18:20:51.579Z caller=log.go:168 level=info msg="Listening on :9219"
^Csignal: interrupt

$ go run ssl_exporter.go --log.format=json
{"caller":"log.go:168","level":"info","msg":"Starting ssl_exporter (version=, branch=, revision=)","ts":"2021-06-10T20:11:19.128Z"}
{"caller":"log.go:168","level":"info","msg":"Build context (go=go1.15.13, user=, date=)","ts":"2021-06-10T20:11:19.129Z"}
{"caller":"log.go:168","level":"info","msg":"Listening on :9219","ts":"2021-06-10T20:11:19.129Z"}
^Csignal: interrupt

Sending a log to a specific level requires to use the go-kit/log/level module with the following syntax: level.<Level>(logger).Log(...).

go-kit/log provides structured logs and I tried to use existing keys that were already used.

[ribbybibby]

Thanks for this! I've left a few comments but lgtm in general.

[johanfleury]

Thanks for the reviewing so quickly :) I think fixed what you asked.

I created a new file (prober/test.go) to put the newTestLogger function, let me know if that’s ok.