[HDX-491] [HDX-492] Support Structured Search over Span Events

[MikeShi42]

Today if users send a span event with structured properties, we don't have a mechanism to allow them to search for it in a structured manner.

Hello, how could I search for span event with level WARN ? level:warm does not return any result. To make it extra clear, I have a row with level ok, but when inspecting this span inside I have a span event with level WARN, this is what I would like my dashboard to show.

https://discord.com/channels/1149498480893640706/1149500035403350036/1204801958809047120

_HDX-492