add attributes support

[agardnerIT]

This PR:

• Adds tracepusher support for sending span attributes

TODO (seperate PR)

The OpenTelemetry docs only show stringValue as a possible type, but hint at floats, ints and arrays being allowed too.

• Asked in Slack. Wait for a resolution to this thread
• Update code + readme

[agardnerIT]

From Georg Pirklbauer on Slack:

TL;DR: stringValue, boolValue, intValue, doubleValue, arrayValue, kvlistValue, bytesValue, with arrayValue and kvlistValue again being able to contain all of the aforementioned types.