Make call to set_attributes to match the OTLP specs

[leggebroten]

Currently, the result of Attributes.get_attributes returns a Keyword which is then passed directly to Tracer.set_attributes.

This is not consistent with the spec which expects a list of two element tuples, {attribute_name_as_binary, OTLP_acceptable_value}

Where a OTLP_acceptable_value is: string, integer, double or boolean

This could lead to integration issues with certain providers.

Example

[{"/http/user_agent" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36"}
 {"/http/server_latency", 300}
 {"abc.com/myattribute", True}
 {"abc.com/score", 10.239}]

[marcdel]

Good catch! Fixed in 2f5ac96