otelhttptrace: allow tracing by adding events

[tonistiigi]

In OpenTracing the equivalent httptrace implementation worked by adding logs(events in opentelemetry) to the parent span https://github.com/opentracing-contrib/go-stdlib/blob/00fa856341a83c0ee331ef758856ad41fdabbea7/nethttp/client.go#L286-L292 while otelhttptrace creates new spans.

In an app that creates a lot of traces, this creates a lot of noise and in my case, I would prefer the timestamped logs. Assuming some users prefer spans, can we make this configurable. For example, variadic Option field to NewClientTrace.