perf: Avoid allocations in OpenTelemetryExtensions

[jamescrosswell]

perf: intermediate string allocations

I know this is just a re-format of existing code.
But I noticed an extra string allocations just used as intermediate data for converting from SpanId to ActivitySpanId, and from SentryId to ActivityTraceId:

• allocating from SpanId/SentryId to string via ToString()
• converted allocation-free to a ReadOnlySpan<char>
• where ActivitySpanId.CreateFromString and ActivityTraceId.CreateFromString both internally create a new string that is passed to the ActivitySpanId/ActivityTraceId constructors

If we would - instead of converting to a heap-allocated string and calling CreateFromString,
would instead either

• format to a stack-allocated Span<char> and call CreateFromString
• format to a stack-allocated ReadOnlySpan<byte> and call CreateFromUtf8String
• format to a stack-allocated ReadOnlySpan<byte> and call CreateFromBytes

we should be able to get rid of half of the heap allocations there.

This is a smaller gain, where we could get, in the SentryId case

• from 176 B (GUID to String with Length 32 and 24 B Class overhead times two)
• to 88 B (cutting above in half)

Do you think this is worth a future improvement?
If so, then I will create a new issue from this comment, labeled Good First Issue with a low priority.
Otherwise, we just resolve this comment.

Originally posted by @Flash0ver in #4283 (comment)

[Flash0ver]

Alongside the existing ToString overrides, we could consider implementing the interfaces

• System.ISpanFormattable
• System.IUtf8SpanFormattable

for SpanId and SentryId, so that the heap-alloc-free implementation may be idiomatically reused in other locations too.