Add "net/rpc" and `database/sql` support during GSoC 2017

[starsdeep]

Currently, instrumentation is just provided for net/http. Instrumentation for net/rpc and database/sqlseems also worth doing.

I am a master who want to apply GSoC 2017, I 'd like to implement the instrumentation for net/rpc or database/sql as my GSoC project.

cc @pritianka

[starsdeep]

ref https://github.com/opentracing/opentracing-go/issues/132

[bhs]

Naively, I'd vote for database/sql over net/http for sure. That said, the problem with the database package is that there's no support for Go Context, and thus it's almost impossible to find a parent SpanContext.

[noxiouz]

Since 1.8 it supports

[bhs]

@noxiouz ah, that's great. I was looking at https://golang.org/pkg/database/sql/ and forgot it's still 1.7. In that case, database/sql SGTM. :)

[zhouheng92]

IMOnet/http/cgi and net/http/fcgi instrumentation is also worth doing. A lot of existing system still use CGI.

[starsdeep]

Will OpenTracing participate GSoC this year? Seem there is no OpenTracing related ideas in the cncf ideas list for GSoC 2018(see https://github.com/cncf/soc)

cc @pritianka