Support Datadog sample rate with global trace sampling from configmap

[sungmincs]

What this PR does / why we need it:
I have been looking for a way to do a Datadog trace sampling in a global level, and ended up filing a PR.

Currently dd-opentracing-cpp has a capability to disable priority sampling (client-side sampling) and use global sampling rate, however there is no native way for ingress-nginx to support this.
Please see the Annotated Datadog config JSON section.
https://github.com/DataDog/dd-opentracing-cpp/blob/7d58654a41a4fa4589988d105f953049f9eeee3c/README.md#guide

In this PR, I added the following:

• support datadog-priority-sampling and datadog-sample-rate in configmap
• Added missing documentations about datadog configuration options
• Added the docs to third-party-addons

I tested the change in my local development.

• Hit an endpoint every 2~3 seconds
• Disable datadog-priority-sampling in configmap
• Change the values of datadog-sample-rate from configmap and observe the difference
  

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):
No known open issues that I know of

Special notes for your reviewer:
Probably useful to get some eyes of dd-opentracing-cpp maintainer, so cc-ing @cgilmour here as this issue was my starting point of this PR.

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[k8s-ci-robot]

Hi @dmxlsj. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[aledbf]

/ok-to-test

[aledbf]

@dmxlsj please squash the commits

[codecov-io]

Codecov Report

Merging #4897 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4897   +/-   ##
======================================
  Coverage    58.5%   58.5%           
======================================
  Files          88      88           
  Lines        6719    6719           
======================================
  Hits         3931    3931           
  Misses       2359    2359           
  Partials      429     429

Impacted Files	Coverage Δ
internal/ingress/controller/nginx.go	29.69% <ø> (ø)	⬆️

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[sungmincs]

sorry messed up the rebase as I was originally working out of on 0.25 release.
will fix this soon.

[sungmincs]

/retest

[sungmincs]

/retest

[cgilmour]

My feedback (dd-opentracing-cpp maintainer) is that it looks correct (code, docs and comments) and the graph from testing show it clearly working as intended.

[aledbf]

@cgilmour thank you for the review!

[aledbf]

/lgtm

[aledbf]

@dmxlsj thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, dmxlsj

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