[trace view] Merged RPC spans

[yurishkuro]

We already have a feature that allows to collapse client-server (RPC) spans into one view, e.g.

However, there are a couple problems with the current implementation:

• This view is only available when the parent span is fully collapsed. Ideally we want this merged display to be independent of the collapsing behavior, because one cannot see the children spans of the server, meaning this merged RPC view is rarely useful
• When the client/server spans are merged, we still need to be able to see full details about each span. In the earlier discussions we decided that it's best to just stack client & server span details on top of each other when the merged span is expanded.
• The text describing the operation names does not fit in the allotted space, it should have a mouseover tip to show the full service::operation strings, or have that displayed in some other way