Add tracing support for Knative Eventing

[boramalper]

Work in progress.

Notes:

• Using golang:1.16-buster as base image for both the builder and the final, since both Alpine and scratch err with:

  standard_init_linux.go:195: exec user process caused "no such file or directory"
  

  because /lib64/ld-linux-x86-64.so.2 was missing:

  $ readelf -l /app/exe | grep "program interpreter"
       [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]

• Added .dockerignore to the root of the repo to exclude bin and .git directories from docker build context; dramatic speed-up.
• As before, consumer is still the pain-point... Still two separate traces. Producer's trace now correctly displays the producer container that is instrumented by us, however consumer's trace is still missing the consumer container (and again, detached from producers trace).
  • Producer's trace: producer-trace.zip
  • Consumer's trace: consumer-trace.zip

[boramalper]

Also see #299.