Draft: Add Clickhouse support to GitLab

What does this MR do and why?

This adds ActiveRecord support for Clickhouse with Rails' multiple database support. Load balancing needs to be disabled since Clickhouse doesn't use the PostgreSQL adapter.

Note that because Clickhouse is a columnar store, the schema used for error tracking needs to be changed to take advantage of this. That means we should probably have:

1. A partitioned table that holds all raw events (no JOIN needed between error_tracking_errors and error_tracking_error_events).
2. A materialized view that aggregates the events.

Addresses: #356481 (closed)

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How to set up and validate locally

Testing locally

config/database.yml

development:
  ...
  clickhouse:
    adapter: clickhouse
    database: gitlabhq_clickhouse_development
    host: localhost
    port: 8123
    username: default
    password:
    ssl: false # optional for using ssl connection
    debug: true # use for showing in to log technical information
    migrations_paths: db/clickhouse/migrate # optional, default: db/migrate_clickhouse
    # cluster_name: 'cluster_name' # optional for creating tables in cluster 
    # replica_name: '{replica}' # replica macros name, optional for creating replicated tables

test: &test
  ...
  clickhouse:
    adapter: clickhouse
    database: gitlabhq_clickhouse_test
    host: localhost
    port: 8123
    username: default
    password:
    ssl: false # optional for using ssl connection
    debug: true # use for showing in to log technical information
    migrations_paths: db/clickhouse/migrate # optional, default: db/migrate_clickhouse
    # cluster_name: 'cluster_name' # optional for creating tables in cluster 
    # replica_name: '{replica}' # replica macros name, optional for creating replicated tables

⚠ No SSL!

Start clickhouse server via Docker

In GDK root (../gitlab)

docker run  -d -p 8123:8123 --name gdk-clickhouse --ulimit nofile=262144:262144 --volume=`pwd`/clickhouse:/var/lib/clickhouse clickhouse/clickhouse-server

⚠ Volume directory clickhouse/ is owned by root on Linux

Migration and testing

rake db:create
rake db:migrate

bin/rails console

payload = JSON.parse(File.read("spec/fixtures/error_tracking/parsed_event.json"))
ErrorTracking::ErrorEvent.create!(payload: json, description: "first", occurred_at: Time.now, actor: "foo", platform: "bar", name: "baz");

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.