fix metric nil pointer panic

[LiZhenCheng9527]

What type of PR is this?
/kind bug

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #703

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.37%. Comparing base (42f3aaa) to head (433592b).
Report is 6 commits behind head on main.

Files	Coverage Δ
pkg/controller/telemetry/metric.go	56.36% <100.00%> (+4.76%)	⬆️

... and 2 files with indirect coverage changes

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[LiZhenCheng9527]

/cc @hzxuzhonghu

[hzxuzhonghu]

should add a test

[hzxuzhonghu]

/lgtm

[kmesh-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hzxuzhonghu

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• pkg/controller/OWNERS [hzxuzhonghu]

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[hzxuzhonghu]

/lgtm