Enable tracing of Cassandra queries

[yurishkuro]

Which problem is this PR solving?

Sometimes searching for spans takes a while, it is interesting to see where the time is spent, since the search in Cassandra is multi-step process.

Short description of the changes

• modify SpanReader API to accept Context in all methods
• create spans inside Cassandra span reader

Example:

[codecov]

Codecov Report

Merging #1038 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1038   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         139     139           
  Lines        6419    6452   +33     
======================================
+ Hits         6419    6452   +33

Impacted Files	Coverage Δ
plugin/storage/memory/memory.go	100% <100%> (ø)	⬆️
plugin/storage/cassandra/spanstore/reader.go	100% <100%> (ø)	⬆️
cmd/query/app/handler.go	100% <100%> (ø)	⬆️
plugin/storage/es/spanstore/reader.go	100% <100%> (ø)	⬆️
storage/spanstore/metrics/decorator.go	100% <100%> (ø)	⬆️

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[jpkrohling]

I'd consider spending a few more minutes thinking about the operation names, but having them follow the method then are in is also OK.

LGTM.