Document how the metrics app is deployed

INSTALL.md

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+# Metrics deployment instructions
+## Create app
+```bash
+dokku$ dokku apps:create metrics
+dokku$ dokku git:set metrics deploy-branch main
+```
+
+## Configure app
+```bash
+dokku config:set metrics GITHUB_TOKEN'xxx'
+dokku config:set metrics SLACK_SIGNING_SECRET='xxx'
+dokku config:set metrics SLACK_TECH_SUPPORT_CHANNEL_ID='xxx'
+dokku config:set metrics SLACK_TOKEN='xxx'
+dokku config:set metrics TIMESCALEDB_URL='xxx'
+```
+
+## Disable checks
+Dokku performs health checks on apps during deploy by sending requests to port 80.
+This tool isn't a web app does it can't accept requests on a port.
+Disable the checks so deploys can happen.
+```bash
+dokku$ dokku checks:disable metrics
+```
+
+## Set up storage
+This is only needed for backfills which make use of a SQLite db cache.
+```bash
+# the metrics container runs as uid ??? (metrics)
+# the other dokku containers on dokku3 run as uid 1013 (dokku)
+# let's tell the container to run as 1013, then we can use the same file permissions
+dokku$ dokku docker-options:add grafana deploy "--user 1013"
+dokku$ dokku docker-options:add grafana run "--user 1013"
+
+dokku$ dokku storage:ensure-directory metrics
+dokku$ sudo chown -R dokku:dokku /var/lib/dokku/data/storage/metrics/
+dokku$ dokku storage:mount metrics /var/lib/dokku/data/storage/metrics/:/storage
+```
+
+## Manually pushing
+```bash
+local$ git clone [email protected]:ebmdatalab/metrics.git
+local$ cd metrics
+local$ git remote add dokku dokku@MYSERVER:metrics
+local$ git push dokku main
+```
+
+## Ensure persistent logs
+```bash
+dokku$ sudo mkdir -p /var/log/journal
+```