We have a large number of classes that track tab state and tab interactions.
Together these report a large number of metrics via UMA and UKM.

So we have multiple classes keeping track of tab stats, each recording different
stats in different ways. There's a resource_coordinator::TabStats struct that
holds per-tab stats, and a metrics::TabsStats struct that holds global
cumulative tab counts. Different tab metrics are uploaded from classes in:
  * chrome/browser/metrics
  * chrome/browser/resource_coordinator
  * chrome/browser/ui
  * chrome/browser/ui/tabs

These trackers are all useful, but there are too many of them to keep track of.
I'm guessing some code for gathering these metrics is redundant, and that some
metrics have become forgotten. It's difficult to figure out where new metrics
should go, or which tracker class to add new tab state info to.

The motivation for this bug is that we want to log considerably more tab metrics
than we currently do, so a sustainable and maintainable model is critical. I'm
happy to take point on this and ensure our existing code is integrated in a way
that we can all agree on.

As part of this, I'll create a list of metrics we currently record to ensure
they are all still relevant and useful.

I'm NOT intending to change or remove every class mentioned below, just listing
them here. We'll consolidate the ones that make sense to be together.



Generally:

* UI and Views code should only be recording tab metrics relating to UI
  affordances, e.g. how often a new tab is created via the context menu or
  tabstrip button. This should happen naturally when the UI is invoked.

* Changes modeled at the UI layer should be reported when the model changes,
  like how BrowserList reports the "ActiveBrowserChanged" action in
  BrowserList::SetLastActive, or how chrome::RestoreTab() reports the
  "RestoreTab" action.

* Trackers in chrome/browser/metrics and chrome/browser/resource_coordinator
  should be merged into a single tab tracker, instead of using purpose-built
  trackers for memory management and metrics; this should be exposed to
  chrome/browser/metrics and chrome/browser/resource_coordinator.

* The tab tracker should be separate from the data store implementation and the
  metrics uploader. We should be able to easily change how metrics are reported
  or how tab stats are persisted without modifying the code that tracks tab
  activity.



Summary of existing classes that I've found so far:

resource_coordinator::TabManager [1]
  * Monitors memory pressure, discards tabs, delays background tab navigations
  * Observes TabStripModel using a BrowserTabStripTracker
  * Observes tab discards via TabLifetimeObserver

resource_coordinator::TabManagerStatsCollector: "records UMAs during session
restore or background tab opening session" [2]
  * Logs histograms and UKMs during session restore and background tab opening
  * Observes SessionRestore
  * Owned by resource_coordinator::TabManager
  * Uses resource_coordinator::TabStats to store per-tab stats

metrics::TabReactivationTracker: "track deactivation/reactivation cycle" [3]
  * Provides a delegate for tab reactivation/deactivation
  * Observes TabStripModel using a BrowserTabStripTracker
  * Used and owned by metrics::TabUsageRecorder
  * Subclass WebContentsHelper stores closing and deactivation status

metrics::TabUsageRecorder: "record metrics about tab reactivation" [4]
  * Logs histograms for tab state (pinned, important) on tab
    deactivation/reactivation events
  * Observes TabStripModel using a BrowserTabStripTracker
  * Observes tab de/reactivation using a metrics::TabReactivationTracker
  * Global instance created by ChromeBrowserMainExtraPartsMetrics after startup,
    never deleted
  * Subclass WebContentsData stores pinned state and inactivity time

metrics::TabStatsTracker: "tracking and recording the tabs and browser windows
usage" [5]
  * Logs daily cumulative tab/windows counts
  * Global instance created by ChromeBrowserMainExtraPartsMetrics after startup,
    never deleted
  * Observes TabStripModel of each Browser
  * Observes BrowserList using a chrome::BrowserListObserver
  * Subclass UmaStatsReportingDelegate reports cumulative tab/windows counts

metrics::TabStatsDataStore: "keeps track of all the information that the
TabStatsTracker wants to track" [6]
  * Owns and exposes a metrics::TabStats to store global cumulative tab counts
  * Updates metrics::TabStats info on demand
  * Persists metrics::TabStats stats in the PrefStore for daily uploading

TabStripModelStatsRecorder: "records user tab interaction stats" [7]
  * Logs UMA histograms when tabs are activated/deactivated/closed
  * Owned by UMABrowsingActivityObserver
  * Observes TabStripModel using a BrowserTabStripTracker
  * Subclass TabInfo stores active/inactive state and creation time
  * Supposed to match the Android implementation in TabUma.java.

UMABrowsingActivityObserver: "watches for loads and creates histograms of some
global object counts" [8]
  * Logs global counts of RenderProcessHosts and per-browser tab counts upon
    every page load
  * Global instance created by ChromeBrowserMainParts at startup, deleted at
    shutdown
  * Observes navigation events and Chrome shutdown



Please let me know if I missed any...

[1] chrome/browser/resource_coordinator/tab_manager.h
[2] chrome/browser/resource_coordinator/tab_manager_stats_collector.h
[3] chrome/browser/metrics/tab_reactivation_tracker.h
[4] chrome/browser/metrics/tab_usage_recorder.h
[5] chrome/browser/metrics/tab_stats_tracker.h
[6] chrome/browser/metrics/tab_stats_data_store.h
[7] chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h
[8] chrome/browser/ui/uma_browsing_activity_observer.h