add build tag to disable tracing

[rashmi-tondare]

This PR addresses issue #704

Changes

Add a build tag retrynotrace to disable tracing to help with dead code elimination

Verification

Wrote a small main function and built it with & without the build tag to get the expected output.

• Without the build tag:

✗ go build -ldflags=-dumpdep ./interceptors/retry/main |& whydeadcode
text/template.(*state).evalField reachable from:
         text/template.(*state).evalFieldChain
         text/template.(*state).evalCommand
         text/template.(*state).evalPipeline
         text/template.(*state).walk
         text/template.(*Template).execute
         html/template.(*Template).Execute
         golang.org/x/net/trace.RenderEvents
         golang.org/x/net/trace.Events
         golang.org/x/net/trace.Events·f
         golang.org/x/net/trace.init.0
         golang.org/x/net/trace..inittask
         go:main.inittasks
         _

• With the build tag (no output):
  ✗ go build -tags=retrynotrace -ldflags=-dumpdep ./interceptors/retry/main |& whydeadcode

[johanbrandhorst]

Could we add a README.md to the interceptors/retry directory that explains how to use this build tag and when it might be useful? Otherwise this looks great!

[rashmi-tondare]

Thanks for your inputs @johanbrandhorst. Will add a readme and update this PR shortly as suggested.

[rashmi-tondare]

@johanbrandhorst i've added a readme as suggested. Please do take a look when you have some time

[johanbrandhorst]

Thanks!