Add doc for opentracing support

[vthiery]

Mention the fact that the client must be initialized with an opentracing interceptor to allow the propagation of spans through the gateway.

I originally followed the doc but completely forgot about adding this interceptor to the gRPC client. Thought it might help others save time.
Please let me know if this isn't accurate enough (or wrong!).

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[codecov-io]

Codecov Report

Merging #987 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #987   +/-   ##
=======================================
  Coverage   53.43%   53.43%           
=======================================
  Files          40       40           
  Lines        4027     4027           
=======================================
  Hits         2152     2152           
  Misses       1676     1676           
  Partials      199      199

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[vthiery]

@googlebot I signed it!

[vthiery]

Having more doc on this topic was actually suggested here

[johanbrandhorst]

Brilliant, thanks for this!

[johanbrandhorst]

Not sure what's up with Googlebot, @achew22?

[vthiery]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[vthiery]

Not sure what's up with Googlebot, @achew22?

I signed the CLA logged with one Google account, but the commit was made with another one. All good now