Feat: Make `write_rfc3339` public or add `Fixed::RFC3339Secs(SecondsFormat)`

[tgrushka]

Only similar issue I found is closed #1339.

The RFC3339 does not require microsecond precision!

• Why not make write_rfc3339 public so we can reuse it?
• Or, add a variant: Fixed::RFC3339Secs(SecondsFormat) that passes SecondsFormat to write_rfc3339 internally (that would actually be really easy to do)?

I want to offer easier seconds formatting for better_tracing to make configuring time easier. The only option from the upstream tracing_subscriber is to use a string to format, much like #1339 suggests.

It's kind of infuriating to have to re-invent the wheel and look up the correct strftime format for the whole thing just to change the number of digits on seconds. In development, who needs microseconds precision 90% of the time?

I'm making public and adding a variant to ChronoFmtType:

#[derive(Debug, Clone, Eq, PartialEq, Default)]
pub enum ChronoFmtType {
    /// Format according to the RFC 3339 convention.
    #[default]
    Rfc3339,
    /// Format with [`SecondsFormat`] seconds format according to the RFC 3339 convention.
    Rfc3339Secs(SecondsFormat),
    /// Format according to multiple `chrono` format [Items][`Item`].
    Items(Vec<Item<'static>>),
    /// Format according to a custom format string.
    Custom(String),
}

and noticed that rfc3339 is just:

            ChronoFmtType::Rfc3339 => {
                write!(
                    w,
                    "{}",
                    t.format_with_items(core::iter::once(Item::Fixed(Fixed::RFC3339)))
                )
            }

But then Fixed::RFC3339 uses write_rfc3339 internally.

So why not Fixed::RFC3339Secs(SecondsFormat) or allow us access to write_rfc3339? Because the feature is already there, it's just not public.

[tgrushka]

Oh wait, I found the solution:

            ChronoFmtType::Rfc3339Secs(secs_fmt) => {
                write!(
                    w,
                    "{}",
                    t.to_rfc3339_opts(*secs_fmt, true)
                )
            }

But this wasn't obvious. Had to "discover" it. May be helpful to make that method public anyway, or add docs to suggest how to achieve the same result.

[djc]

Open to reviewing a PR for this.