fix: avoid possible panic in govc metric commands

[dougm]

Description

Newer versions of vCenter may return a CounterId via QueryAvailablePerfMetric()
which does not have a corresponding PerfCounterInfo entry in PerformanceManager's
perfCounter property. This results in a panic by a few of the govc metric commands
and any application using performance.Manager.AvailableMetric with Sort enabled.

While this may be a bug in vCenter, it is included in some recent release versions.
This workaround avoids the panic by ignoring any CounterId w/o a PerfCounterInfo.

Also updated vcsim's PerformanceManager.QueryCounter to match real vCenter's behavior.
Where null is returned for an unknown CounterId.

Closes #2835

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

How Has This Been Tested?

Tested against vCenter build 55745184 (unreleased 8.0).
Updated vcsim to add this behavior, which triggered the same panics in govc metric commands without this fix.

Checklist:

• My code follows the CONTRIBUTION guidelines of
  this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[embano1]

LGTM, just not sure if this one broke the govc test?

[dougm]

LGTM, just not sure if this one broke the govc test?

#2850 broke the vm.migrate test, I will look into that and we can fix in a separate PR. Hadn't seen vcsim examples tests fail that way before, seems related to the Go install on the action workers.

[embano1]

Ah, I have seen those before. It's due to some variables we use in the Makefile which shadow certain Go installation variables/GOROOT in the action runner. Need to look into that.

[dougm]

@embano1 PR #2867 fixes the govc vm.migrate test

[embano1]

LGTM