[7.x] Document api_key in apm-server.yml file (#3138)#3173

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simitt merged 1 commit intoelastic:7.xfrom simitt:backport/7.x/pr-3138

Jan 15, 2020

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Commits on Jan 15, 2020

Document api_key in apm-server.yml file (elastic#3138 )
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