[tracing appender] Collect span attributes

[krfricke]

Seems related to #1378

Changes

Currently, span attributes are not collected by the OpenTelemetryTracingBridge. E.g. when using

        let my_span =
            tracing::info_span!("my_span", foo = "bar");

        my_span.in_scope(|| {
            tracing::info!(baz = 42, "Log message with metrics");
        });

this will emit the log message with attribute baz = 42, but without foo = bar.

It can still be desirable to emit attribute values, e.g. when not correlating to emitted tracing data.

This PR adds such functionality by adding two flags that control if span attributes are emitted with logs, and if so, if they are flattened or nested.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

• ❌ - login: @krfricke / name: Kai Fricke . The commit (828354f, 279061a) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[krfricke]

I'm reviewing the CLA with my company.

[cijothomas]

I'd like to hold off from making further changes in this crate until #1689 is resolved to avoid rework.

I do not disagree about this change itself, and we can consider it after the higher order bits are resolved.

[krfricke]

Sounds good, please feel free to ping me on this PR once that's in place.

[cijothomas]

Sounds good, please feel free to ping me on this PR once that's in place.

Thanks.
While you wait, we are glad to receive other contributions! Feel free to take a look around and see if you want to help with something.

Since you already have experience with tracing, would be good get your thoughts on #1689 as well!