Merge branch 'am-add-snowplow-debugging-page-to-docs' into 'master'

Add troubleshooting page to Snowplow guide

See merge request gitlab-org/gitlab!78666

doc/development/snowplow/troubleshooting.md

+---
+stage: Growth
+group: Product Intelligence
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Troubleshooting
+
+## Good events drop
+
+### Symptoms
+
+You will be alarmed via a [Sisense alert](https://app.periscopedata.com/app/gitlab/alert/Volume-of-Snowplow-Good-events/5a5f80ef34fe450da5ebb84eaa84067f/edit) that is sent to `#g_product_intelligence` Slack channel
+
+### Locating the problem
+
+First you need to identify at which stage in Snowplow the data pipeline the drop is occurring.
+Start at [Snowplow dashboard](https://console.aws.amazon.com/systems-manager/resource-groups/cloudwatch?dashboard=SnowPlow&region=us-east-1#) on CloudWatch,
+if you do not have access to CloudWatch you need to create an [access request issue](https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/9730) first.
+While on CloudWatch dashboard set time range to last 4 weeks, to get better picture of system characteristics over time. Than visit following charts:
+
+1. `ELB New Flow Count` and `Collector Auto Scaling Group Network In/Out` - they show in order: number of connections to collectors via load balancers and data volume (in bytes) processed by collectors. If there is drop visible there, it means less events were fired from the GitLab application. Proceed to [application layer guide](#troubleshooting-gitlab-application-layer) for more details
+1. `Firehose Records to S3` - it shows how many event records were saved to S3 bucket, if there was drop on this chart but not on the charts from 1. it means that problem is located at AWS infrastructure layer, please refer to [AWS layer guide](#troubleshooting-aws-layer)
+1. If drop wasn't visible on any of previous charts it means that probelm is at data warehouse layer, please refer to [data warehouse layer guide](#troubleshooting-data-warehouse-layer)
+
+### Troubleshooting GitLab application layer
+
+Drop occurring at application layer can be symptom of some issue, but it might be also a result of normal application lifecycle, intended changes done to product intelligence or experiments tracking
+or even a result of a public holiday in some regions of the world with a larger user-base. To verify if there is an underlying problem to solve, you can check following things:
+
+1. Check `about.gitlab.com` website traffic on [Google Analytics](https://analytics.google.com/) to verify if some public holiday might impact overall use of GitLab system
+   1. You may require to open an access request for Google Analytics access first eg: [access request internal issue](https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/1772)
+1. Plot `select date(dvce_created_tstamp) , event , count(*) from legacy.snowplow_unnested_events_90 where dvce_created_tstamp > '2021-06-15' and dvce_created_tstamp < '2021-07-10' group by 1 , 2 order by 1 , 2` in SiSense to see what type of events was responsible for drop
+1. Plot `select date(dvce_created_tstamp) ,se_category , count(*) from legacy.snowplow_unnested_events_90 where dvce_created_tstamp > '2021-06-15' and dvce_created_tstamp < '2021-07-31' and event = 'struct' group by  1 , 2 order by  1, 2` what events recorded the biggest drops in suspected category
+1. Check if there was any MR merged that might cause reduction in reported events, pay an attention to ~"product intelligence" and ~"growth experiment" labeled MRs
+1. Check (via [Grafana explore tab](https://dashboards.gitlab.net/explore) ) following Prometheus counters `gitlab_snowplow_events_total`, `gitlab_snowplow_failed_events_total` and `gitlab_snowplow_successful_events_total` to see how many events were fired correctly from GitLab.com. Example query to use `sum(rate(gitlab_snowplow_successful_events_total{env="gprd"}[5m])) / sum(rate(gitlab_snowplow_events_total{env="gprd"}[5m]))` would chart rate at which number of good events rose in comparison to events sent in total. If number drops from 1 it means that problem might be in communication between GitLab and AWS collectors fleet.
+1. Check [logs in Kibana](https://log.gprd.gitlab.net/app/discover#) and filter with `{ "query": { "match_phrase": { "json.message": "failed to be reported to collector at" } } }` if there are some failed events logged
+
+For results about an investigation conducted into an unexpected drop in snowplow events volume, see [this issue](https://gitlab.com/gitlab-org/gitlab/-/issues/335206).
+
+### Troubleshooting AWS layer
+
+Already conducted investigations:
+
+- [Steep decrease of Snowplow page views](https://gitlab.com/gitlab-org/gitlab/-/issues/268009)
+- [`snowplow.trx.gitlab.net` unreachable](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5073)
+
+### Troubleshooting data warehouse layer
+
+Reach out to [Data team](https://about.gitlab.com/handbook/business-technology/data-team) to ask about current state of data warehouse. On their handbook page there is a [section with contact details](https://about.gitlab.com/handbook/business-technology/data-team/#how-to-connect-with-us)