Standardize log format of relay proxy

[launchdarkly]

Standardize log format of relay proxy

[eli-darkly]

Ah no, I spoke too soon, it's a pretty easy fix after all.

[eli-darkly]

In the upcoming release, we're planning to standardize the format as follows:

2020/01/06 18:25:59.404164 [env: my-environment-name] DEBUG: Received all feature flags

Messages that aren't tied to a specific environment would omit the [env: xxx] field - although for simplicity in parsing I guess we could instead substitute something like [main]. (Edited to add: the latter.)

[eli-darkly]

Good point. This wasn't an intentional difference; shouldn't be hard to fix.

[eli-darkly]

Oh... I see why it's that way. Ugh. Go's built-in Logger does not have any notion of "levels", and there is no way to make it prepend a string like "DEBUG:" to every log message that you write; all you can do is prepend a string to the whole line. That's not a desirable format— it would be much better to put the timestamp first— but as long as the loggers are implemented as a log.Logger, we have no other way to make "DEBUG:", "ERROR:", etc. show up unless we hard-code them into every log string. And because Relay is usable as a library inside an application, and those logger instances are exported global variables, changing their type would be a breaking change. So... we may have to just hard-code those prefixes for now. It's clunky but I think it's worth it to make the timestamp be at the beginning of the line.

[eli-darkly]

The 5.9.2 release includes this change.