Do not normalize keys in JoinTraceFromText
Do not normalize keys in JoinTraceFromText

The documentation for SetTraceAttribute[1] states:

    As such, `restrictedKey` MUST match the regular expression
    `(?i:[a-z0-9][-a-z0-9]*)` and is case-insensitive. [...]
    If `restrictedKey` does not meet these criteria, SetTraceAttribute()
    results in undefined behavior.

This change removes the ad-hoc normalization (lower-casing)
of attribute keys in JoinTraceFromText. It isn't done for
the binary serialization, and it seems wasteful to put an
unconditional penalty in the hot path which attempts to
partially salvage undefined behavior.

[1]: https://godoc.org/github.com/opentracing/opentracing-go#Span

opentracing/basictracer-go