derp: add metrics to server got pings, sent pongs

Updates #3652

Change-Id: I1d350bcaee39ea36b0c71912028624d18fb541b4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f