Clarify where to use NewHandler()

[gebn]

The docs for this function suggest NewHandler() should wrap a mux, and WithRouteTag() individual routes on that mux. However, this forces a single operation for every path, which sounds wrong. This is causing some confusion, as several reputable sites (1, 2, 3 - the first one written by a core maintainer of OTel) teach wrapping routes with NewHandler().

[wolverian]

The semantics of the operation parameter are unclear to me as well. What does it mean? For example, on this line:

https://github.com/open-telemetry/opentelemetry-go-contrib/blob/v1.3.0/instrumentation/net/http/otelhttp/handler.go#L127

operation is passed to semconv. semconv.HTTPServerAttributesFromHTTPRequest as the serverName parameter:

https://github.com/open-telemetry/opentelemetry-go/blob/v1.3.0/semconv/v1.7.0/http.go#L199