model: add "classname" to stacktrace frames

[axw]

This will be used by the OpenTelemetry exporter.
We should probably be using this for named type
methods in the Go Agent too, but that can come
later.

[apmmachine]

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• Build Cause: Pull request #862 opened

• Start Time: 2020-12-17T09:44:49.834+0000

• Duration: 24 min 45 sec

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Test	Results
Failed	0
Passed	7661
Skipped	188
Total	7849