Update regular expression for traceStateKeyFormatWithMultiTenantVendor

[YuriiBarninets]

The regular expression for tracestate key format with multi-tenant vendor does not match W3C TraceContext specification.

TraceContext specification:
https://www.w3.org/TR/trace-context-1/#key

TraceContext specification allows digits to be placed at the beginning of tracestate key for multi-tenant vendor scenarios, however the current regular expression in Go OpenTelemetry prohibits it:

traceStateKeyFormatWithMultiTenantVendor = [a-z][_0-9a-z\-\*\/]{0,240}@[a-z][_0-9a-z\-\*\/]{0,13}

As a result, the following key will be reinterpreted as the wrong one:

968a934b-980df25b@dt

[MrAlias]

There is some ambiguity with the start character of the key as described in this issue. I think we should update to match what is there as this issues describes, but it should be understood that things might change in the future.

[MrAlias]

Looking into addressing this and adding tests, we should at a minimum test for the same data set that the W3C project does: https://github.com/w3c/trace-context/blob/master/test/test_data.json