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>

flakehashes.json

@@ -4,7 +4,7 @@
     "sri": "sha256-cY5yryX+p/xtoTv+WZEKFagiIl0OREHnJY1Bk5VpVVc="
   },
   "vendor": {
-    "goModSum": "sha256-n22/ZMRK+Wm2z0bdnhKTjC8rO+8KG9wnVYMevQy7lsg=",
-    "sri": "sha256-w6MF0pKV6zfeVorfrxMm2AXR0uDgw5sG3zl5Ugm6SCU="
+    "goModSum": "sha256-huA5ZwAUhTpOS4+iO4copCH7NVRqjgqCKsfBaUvg790=",
+    "sri": "sha256-Lev0BkZu+gX17Wt6Xd3Lh6v0MollbVrE84+ILxAf6aI="
   }
 }