add apm-server

apm-server/data/README.md

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+apm-server
+==========
+
+[Elastic APM][1] is an application performance monitoring system built on the
+Elastic Stack. It allows you to monitor software services and applications in
+real time, collecting detailed performance information on response time for
+incoming requests, database queries, calls to caches, external HTTP requests,
+etc. This makes it easier to pinpoint and fix performance problems quickly.
+
+Elastic APM also automatically collects unhandled errors and exceptions. Errors
+are grouped based primarily on the stacktrace, so you can identify new errors
+as they appear and keep an eye on how many times specific errors happen.
+
+[1]: https://www.elastic.co/guide/en/apm/get-started/current/index.html