go.mod,tsnet,tstest/natlab/vmtest: bump prometheus/common to v0.69.0

prometheus/common v0.66/v0.67 introduced a mandatory
model.ValidationScheme on expfmt.TextParser as part of
prepping for UTF-8 metric/label names in Prometheus 3.0. The
zero value is intentionally UnsetValidation, which panics on
the first call to IsValidMetricName / IsValidLabelName with

  Invalid name validation scheme requested: unset

so the long-standing "var parser expfmt.TextParser" pattern
crashes at runtime. Several big downstreams have hit the same
sharp edge:

  https://github.com/thanos-io/thanos/issues/8823
  https://github.com/grafana/loki/pull/21401

Switch our two callers (parseMetrics in tsnet's
TestUserMetricsByteCounters and the client-metrics scraper in
tstest/natlab/vmtest) to the new expfmt.NewTextParser
constructor with model.LegacyValidation. LegacyValidation
matches the classic ASCII metric/label naming rules that
tailscaled's exporter uses today; if and when we ever emit a
metric with a UTF-8 name, we can revisit.

Goes to v0.69.0 (the latest at the time of writing) rather
than v0.67.5 so we pick up the unrelated security fixes for
cross-host redirects.

Done in advance so a follow-up change can pull in
github.com/tailscale/policybottest (which depends on
palantir/policy-bot, which transitively requires
prometheus/common at v0.67+) without dragging this debugging
into that PR.

Updates tailscale/corp#13972

Change-Id: I4b37db9ad3bebef1a32d9020bf6f8790bab25336
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vmtest/vmtest.go

@@ -41,6 +41,7 @@ import (
 	"github.com/google/gopacket/layers"
 	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/common/expfmt"
+	"github.com/prometheus/common/model"
 	"go4.org/mem"
 	"golang.org/x/sync/errgroup"
 	"tailscale.com/client/local"
@@ -909,7 +910,11 @@ func (e *Env) ClientMetrics(n *Node) ClientMetrics {
 	}
 
 	// Metrics are reported in Prometheus exposition format.
-	var parser expfmt.TextParser
+	// prometheus/common v0.67 made the validation scheme mandatory;
+	// the zero-value parser now panics. LegacyValidation matches the
+	// classic ASCII metric/label name rules that the tailscaled
+	// client exporter uses.
+	parser := expfmt.NewTextParser(model.LegacyValidation)
 	mfs, err := parser.TextToMetricFamilies(bytes.NewReader(raw))
 	if err != nil {
 		e.t.Fatalf("Node %q parse client metrics: %v", n.Name(), err)