add context to custom trace name function to allow passing context va…

[realrunner]

…lues to name queries

[obitech]

Thanks for the PR! On first glance, hiding the sql_name in the context seems to be a bit a "magic" to me and this feature might not be immediately clear to every user. Could you give us a little more context (hehe) and/or an example how you intend to use this feature?

[costela]

I think sql_name is just a test placeholder. AFAICT the idea is to extract the span name from more than just the statement. E.g.: you may have the same statement being called in multiple parts of the code, each with a different context. @realrunner is that assumption correct?

I kinda see the use-case, but I'm a bit skeptical that it's a good trade-off: the big advantage of traces compared to e.g. logs is their hierarchical nature. I'd expect the parent spans to give you more than enough information without having to cram more stuff in the span's name?

[costela]

nit: let's use zero sized structs instead, to save allocations

Suggested change

	type ctxKey string
	type ctxKeySQLName struct{}

(even if this is just a test, it's probably a good idea to keep to good practices)