Update prometheus/common to support --log.format option

[shusugmt]

Support for json format logging in prometheus/common/promlog has been added since prometheus/common#136
This PR updates prometheus/common to the latest version which supports --log.format option.

The update also includes code change to adopt prometheus/common#129
As a result now http prober exposes new metrics described in prometheus/prometheus#3241 via /metrics endpoint.

[brian-brazil]

Thanks for working on this. The newer version of common also makes all http connections use keepalives, which is undesirable for this particular exporter. That'll need work first.

[shusugmt]

I just noticed there are a few similar branches trying to update prometheus/common

• https://github.com/prometheus/blackbox_exporter/tree/fish-update-dependencies
• https://github.com/prometheus/blackbox_exporter/tree/bjk/prom_common

And found this PR #321
Then that leads to prometheus/common#133 which seems blocking this to proceed.

[shusugmt]

The newer version of common also makes all http connections use keepalives, which is undesirable for this particular exporter

Solution seems quite straightforward once prometheus/common#133 got merged. Simply pass true as the third parameter to NewClientFromConfig() in https://github.com/prometheus/blackbox_exporter/pull/393/files#diff-29df7d46fe6b972c1946a34608c58ab3R219

[brian-brazil]

This is obsolete.