Regression: `basic_auth` should not require a `username`

[juliusv]

After #4077, basic_auth in HTTP configs now requires setting a username, otherwise you get an error:

[...] err="Error loading config couldn't load configuration (--config.file=prometheus.yml): parsing YAML file prometheus.yml: basic_auth requires a username"

Using basic auth without a username has been standard practice, for example in the suggested remote_write configs for Cortex. So this is a breaking change and needs to be fixed.

[adamdecaf]

Whoops, prometheus/common#135 fixes that.

[juliusv]

@adamdecaf Awesome, thanks :)

[adamdecaf]

#4174 Should fix it in prometheus itself.

[lock]

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