Read extra http headers config from tracing object of discovery response

[basti1302]

Additional context:

• This is related to this sensor change: https://github.ibm.com/instana/sensors/pull/342 (released via https://github.ibm.com/instana/agent-update-site/pull/574 as discovery/sensor version 1.6.4). This has been rolled out in May already.
• The change in the agent-side sensor as well as in the in-process collector here are backwards compatible. I've tested that the old (< 1.6.4) discovery agent plug-in works with the updated in-process collector code and vice versa (new discovery plug-in 1.6.4 with old in-process collector code). Obviously I also tested the latest discovery version with the updated in-process collector code, that is, with this change.