Automatically recognize SpanStartTimestamp, ParentSpanId, and SpanKind from SerilogTracing

[nblumhardt]

SerilogTracing is a nice low-dependency way to light up Serilog output with timed/hierarchical traces. Its tracing output boils down to three properties, listed in the title.

There's currently a SerilogTracing.Sinks.Seq package that maps these three properties into underlying top-level compact JSON fields to send to Seq.

Since the whole thing is gated on the existence of SpanStartTimestamp, on a log event that is already marked with a trace and span id, adding support directly here seems reasonable: it's unlikely this will break any existing usage, and we'll avoid having to support yet another same-but-not-quite client library for people who want to use Serilog with tracing.

SpanStartTimestamp maps to @st, which Seq already recognizes, and this is the case for ParentSpanId mapping to @ps, also.

The @sk property name is not currently recognized by Seq and will be recorded verbatim; I'm expecting we'll either ignore or ingest this in 2024.2, and we'll have time to validate that decision before this sink reaches 7.0.0.

[nblumhardt]

I've kept to the convention of using string literals in this file, since it makes the code easier to inspect (juggling hard-coded strings is pretty much all this class does :-) )