feat(discover2) Add public fields to discover for tracing

[markstory]

I've not added the new fields to the query builder as I don't think they are that useful to end user result sets. I've opted to go with the trace prefix as that matches the tags we're currently generating and the name of the context in the raw event payloads.

[dashed]

[dashed]

@markstory So I took the liberty to try this PR out, I had assumed it would work.

Fortunately, I am able to query by trace.parent_span.

However, trace and trace.span no longer works 😞 .

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For this query: event.type:transaction trace.span:b91d88975fcd209b

I'm getting

Code: 43.
DB::Exception: Illegal types of arguments (UInt64, String) of function equals. Stack trace:

0. /usr/bin/clickhouse-server(StackTrace::StackTrace()+0x30) [0x7dd0150]
1. /usr/bin/clickhouse-server(DB::Exception::Exception(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)+0x25) [0x3bf89f5]
2. /usr/bin/clickhouse-server(DB::FunctionComparison<DB::EqualsOp, DB::NameEquals>::getReturnTypeImpl(std::vector<std::shared_ptr<DB::IDataType const>, std::allocator<...

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For this query: event.type:transaction trace:2d9582eb6696409c9acddeccaf9dfa1e trace.parent_span:b91d88975fcd209b

I'm getting:

Code: 376.
DB::Exception: Cannot parse uuid 2d9582eb6696409c9acddeccaf9dfa1e. Stack trace:

0. /usr/bin/clickhouse-server(StackTrace::StackTrace()+0x30) [0x7dd0150]
1. /usr/bin/clickhouse-server(DB::Exception::Exception(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)+0x25) [0x3bf89f5]
2. /usr/bin/clickhouse-server(DB::FunctionComparison<DB::EqualsOp, DB::NameEquals>::executeDateOrDateTimeOrEnumOrUUIDWithConstString(DB::Block&, unsigned long, DB::IColumn const*...

[markstory]

Whoops forgot about the column types in snuba. For the time being I'll go back to using contexts as those values are all strings, and make a note to figure out the uuid/int mappings in snuba.