Switch from glog to slog for structured (as well as leveled) logging

[scr-oath]

slog wins

• slog is now part of the SDK, maintained by the language owners
• slog has support for OpenTelemetry bridging
• slog has json format and, can log any structure in attributes
• There are some nice libraries already https://github.com/samber/slog-multi and https://github.com/samber/slog-sampling e.g. to duplicate, route, or sample logs.

glog shortcomings

• glog has issue with write size - it converts newlines to \n for only as big as will fit in its buffer; beyond that, all newlines are as-is and cause multiple, disjoint, possibly interleaved lines to be written.
• glog's format does not support things like json for log collectors (like google cloud observability suite, that can query them with BigQuery log analytics)