Rewrite the friendly log handler

[pietern]

Changes

It wasn't working because it deferred to the regular slog.TextHandler for the WithAttr and WithGroup functions. Both of these functions don't mutate the handler but return a new one. When the top-level logger called one of these, log records in that context used the standard handler instead of ours.

To implement tracking of attributes and groups, I followed the guide at https://github.com/golang/example/blob/master/slog-handler-guide/README.md for writing custom handlers.

Tests

The new tests demonstrate formatting through t.Log and look good.

[pietern]

Output of the slog.TextHandler:

Output of the previous friendly handler (when it worked):

Output of the new friendly handler:

[pietern]

I made the colors a bit more muted compared to the previous friendly logger, to try and emphasize the log message over the log attributes. The current implementation uses the same approach as slog.TextHandler on dealing with grouped attributes, where the group key(s) and attribute keys are separated with a .. Another approach is to turn them into JSON and write this as a formatted block in a muted color as background information (multi-line).

[pietern]

Colors of different log levels:

[pietern]

Handling of (grouped) attributes:

[pietern]

Dark modern:

Light modern:

Solarized dark:

Solarized light:

[pietern]

Tweaked the colors of the message and attributes a bit more.

For multiline messages (like the ones logged by the SDK), the FgWhite color for messages is very in-your-face. The following uses the default color for messages and dims the keys of the attributes to emphasize their value.

Dark modern:

Light modern:

Solarized dark:

Solarized light: