feat: add workspace agent session count protocol (#28125)

Phase 2 of the Scalable Approach for Adding New IDE Session Types RFC,
tracked in #27410.

Adds a session_counts map to Stats on the Agent API, so an agent can report
a count per app name instead of the four fixed fields, which are now marked
deprecated. Bumps the API to v2.11.

- coderd normalizes names on ingest, caps distinct entries, and sums the
  overflow under unknown, so an arbitrary client name cannot blow up
  cardinality.
- Agents predating v2.11 set the fixed fields instead, never both, and the
  ingest path falls back to them.

Nothing sends session_counts yet. The agent starts populating it in #28338.

coderd/agentapi/stats.go

@@ -66,10 +66,7 @@ func (a *StatsAPI) UpdateStats(ctx context.Context, req *agentproto.UpdateStatsR
 		// while the experiment is enabled we will not report
 		// session stats from the agent. This is because it is
 		// being handled by the CLI and the postWorkspaceUsage route.
-		req.Stats.SessionCountSsh = 0
-		req.Stats.SessionCountJetbrains = 0
-		req.Stats.SessionCountVscode = 0
-		req.Stats.SessionCountReconnectingPty = 0
+		workspacestats.ClearSessionCounts(req.Stats)
 	}
 
 	err := a.StatsReporter.ReportAgentStats(

coderd/agentapi/stats_test.go

@@ -525,6 +525,7 @@ func TestUpdateStats(t *testing.T) {
 		batcher.Mu.Lock()
 		defer batcher.Mu.Unlock()
 		require.EqualValues(t, 1, batcher.Called)
+		require.Empty(t, batcher.LastStats.GetSessionCounts())
 		require.EqualValues(t, 0, batcher.LastStats.SessionCountSsh)
 		require.EqualValues(t, 0, batcher.LastStats.SessionCountJetbrains)
 		require.EqualValues(t, 0, batcher.LastStats.SessionCountVscode)