logpolicy: expose certain metrics as clientmetricsdsnet/logpolicy-metrics

This allows us to detect how often we are actually filching
(and therefore whether to maintain this feature).
It also reports the storage size and amount of dropped bytes.

Updates tailscale/corp#21363

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

3 files changed, 42 insertions, 0 deletions

diff --git a/logpolicy/logpolicy.go b/logpolicy/logpolicy.go
index f7491783a..741e02fc7 100644
--- a/logpolicy/logpolicy.go
+++ b/logpolicy/logpolicy.go
@@ -662,6 +662,9 @@ func (opts Options) init(disableLogging bool) (*logtail.Config, *Policy) {
 		}
 	}
 	lw := logtail.NewLogger(conf, opts.Logf)
+	if conf.MetricsDelta != nil {
+		exportClientMetrics(lw)
+	}
 
 	var logOutput io.Writer = lw
 
diff --git a/logpolicy/logtail_metrics.go b/logpolicy/logtail_metrics.go
new file mode 100644
index 000000000..87fa7de89
--- /dev/null
+++ b/logpolicy/logtail_metrics.go
@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_logtail && !ts_omit_clientmetrics
+
+package logpolicy
+
+import (
+	"expvar"
+
+	"tailscale.com/logtail"
+	"tailscale.com/util/clientmetric"
+)
+
+func exportClientMetrics(logger *logtail.Logger) {
+	if m, _ := logger.ExpVar().(interface{ Get(string) expvar.Var }); m != nil {
+		if m2, _ := m.Get("buffer").(interface{ Get(string) expvar.Var }); m2 != nil {
+			if v, _ := m2.Get("counter_filched_bytes").(*expvar.Int); v != nil {
+				clientmetric.NewCounterFunc("logtail_filched_bytes", v.Value)
+			}
+			if v, _ := m2.Get("counter_dropped_bytes").(*expvar.Int); v != nil {
+				clientmetric.NewCounterFunc("logtail_dropped_bytes", v.Value)
+			}
+			if v, _ := m2.Get("gauge_stored_bytes").(*expvar.Int); v != nil {
+				clientmetric.NewGaugeFunc("logtail_stored_bytes", v.Value)
+			}
+		}
+	}
+}
diff --git a/logpolicy/logtail_nometrics.go b/logpolicy/logtail_nometrics.go
new file mode 100644
index 000000000..51ce04689
--- /dev/null
+++ b/logpolicy/logtail_nometrics.go
@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_logtail || ts_omit_clientmetrics
+
+package logpolicy
+
+import "tailscale.com/logtail"
+
+func exportClientMetrics(logger *logtail.Logger) {}