Add APM server extension (#326)

Why:
* Adds Elasticsearch Application Performance Monitoring extension
* Can view application traces, errors and alerts trough Kibana

How:
* Add extension for apm-server container to the network which will receive traffic on port 8200 and report to elastic search server at port 9200

extensions/apm-server/Dockerfile

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+ARG ELK_VERSION
+
+FROM docker.elastic.co/apm/apm-server:${ELK_VERSION}

extensions/apm-server/README.md

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+# APM Server extension
+
+Adds a container for Elasticsearch APM server. Forwards caught errors and traces to Elasticsearch
+server that can be viewed in Kibana. 
+
+## Usage
+
+If you want to include the APM server, run Docker compose from the root of 
+the repository with an additional command line argument referencing the `apm-server-compose.yml` file:
+                                                                           
+```bash
+$ docker-compose -f docker-compose.yml -f extensions/apm-server/apm-server-compose.yml up
+```
+
+## Connecting an agent to APM-Server
+
+The most basic configuration to send traces to apm server. Is to specify the
+`SERVICE_NAME` and `SERVICE_URL`. Here is an example Python FLASK configuration: 
+
+```python
+import elasticapm
+from elasticapm.contrib.flask import ElasticAPM
+
+from flask import Flask
+
+app = Flask(__name__)
+app.config['ELASTIC_APM'] = {
+    # Set required service name. Allowed characters:
+    # a-z, A-Z, 0-9, -, _, and space
+    'SERVICE_NAME': 'PYTHON_FLASK_TEST_APP',
+
+    # Set custom APM Server URL (default: http://localhost:8200)
+    'SERVER_URL': 'http://apm-server:8200',
+
+    'DEBUG': True,
+}
+```
+
+More configuration setting can be found under the **Configuration**
+section for each language. Link: https://www.elastic.co/guide/en/apm/agent/index.html 
+
+## Checking Connectivity and Importing default APM Dashboards
+
+From Kibana main window press:
+
+1. `Add APM` button under Add Data to Kibana section
+2. Ignore all the install instructions and press `Check APM Server status` button.
+3. Press `Check agent status`
+4. Press `Load Kibana objects` to get the default dashboards
+5. Lastly press the `APM dashboard` to the bottom right.
+
+## APM Agent Documentation
+
+Link: https://www.elastic.co/guide/en/apm/agent/index.html

extensions/apm-server/apm-server-compose.yml

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+version: '2'
+
+services:
+  apm-server:
+    build:
+      context: extensions/apm-server/
+      args:
+        ELK_VERSION: $ELK_VERSION
+    volumes:
+      - ./apm-server/config/apm-server.yml:/usr/share/apm-server/config/apm-server.yml:ro
+    ports:
+      - "8200:8200"
+    networks:
+      - elk
+    depends_on:
+      - elasticsearch

extensions/apm-server/config/apm-server.yml

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+apm-server:
+  host: localhost:8200
+
+output:
+  elasticsearch:
+    hosts: http://elasticsearch:9200