Log sleep durations more definitively

[brandur]

While looking into #695 the other day, I was reminded that the handling
of time.Duration in things like logging is potentially not very good.

time.Duration has a good String() override that tends to get used
with text handlers, but for various legacy reasons it doesn't have a
MarshalJSON implementation, so when dumped to JSON it gets put in
nanoseconds, which isn't very readable for human or computer.

This is relevant to use because slog's JSONHandler and TextHandler
will probably be the most common logging instruments for River. e.g.

func main() {
    dur := 500 * time.Millisecond

    loggerText := slog.New(slog.NewTextHandler(os.Stdout, nil))
    loggerText.Info("with text handler", "sleep_duration", dur)

    loggerJSON := slog.New(slog.NewJSONHandler(os.Stdout, nil))
    loggerJSON.Info("with JSON handler", "sleep_duration", dur)
}

time=2024-12-19T21:36:41.948-07:00 level=INFO msg="with text handler" sleep_duration=500ms
{"time":"2024-12-19T21:36:41.949239-07:00","level":"INFO","msg":"with JSONtext handler","sleep_duration":500000000}

Here, change the various places that we log sleep to use a more
definitive value for purposes of the log. In this case a duration string
representation like 10s.

I debated making this a float instead that'd be represented in seconds
because the float would be more parseable for ingestion engines like
Splunk. I went with the former option in the end though because it's
probably better for humans and most other cases, and we could always add
an alternative sleep_seconds field that's a float later if someone
asks for it.