CXXCBC-744: Add request_encoding spans for insert, replace, upsert

[DemetrisChr]

Motivation

KV operations that encode the document content must create a request_encoding span.

Changes

For Insert, Replace, Upsert:

• Defer the encoding of the content until the relevant method of collection_impl is called, which has access to the tracer and creates the top-level span. The insert/replace/upsert methods of collection_impl can now accept an std::function parameter that encodes the document.
• Create a request_encoding span when invoking the function that provides the encoded document.
• If replace/upsert/insert is called with an encoded document, then no request_encoding span is created.