metrics: fix metrics Get Backend Count and Ping Backend Duration

[djshow832]

What problem does this PR solve?

Issue Number: close #279

Problem Summary:

• Get Backend Count shows 2 succeed without instance names
• Ping Backend Duration shows double legends, one is 2s and one is a normal value

What is changed and how it works:

• Add instance name to Get Backend Count
• Remove res label from Ping Backend Duration

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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None