fix: Don’t consider log events as root spans

[MikeGoldsmith]

Which problem is this PR solving?

Log events should never be considered root spans even though they do not contain a trace.parent_id. A log event that is part of a trace would mark the trace complete as if it's considered a root span.

This change updates the check for whether a span is considered a root span to also check for the meta field meta.signal_type. If the field exists and is set to "log", we return false.

Short description of the changes

• Update InMemoryCollector.isRootSpan to check if meta.signal_type and has a value of "log"
• Add unit tests for InMemoryCollector to verify expected behaviour for isRootSpan

[kentquirk]

This is good.