Add metrics for plan-preview component

[nghialv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Some issues were detected while linting go source files in your changes.

[pipecd-bot]

Code coverage for golang is 32.10%. This pull request increases coverage by 0.03%.

File	Function	Base	Head	Diff
pkg/app/piped/planpreview/planpreviewmetrics/metrics.go	ReceivedCommands	--	100.00%	+100.00%
pkg/app/piped/planpreview/planpreviewmetrics/metrics.go	HandledCommand	--	100.00%	+100.00%
pkg/app/piped/planpreview/planpreviewmetrics/metrics.go	Register	--	0.00%	+0.00%
pkg/app/piped/planpreview/handler.go	Handler.enqueueNewCommands	95.24%	95.45%	+0.22%
pkg/app/piped/planpreview/handler.go	Handler.handleCommand	44.83%	45.45%	+0.63%
pkg/app/piped/planpreview/handler.go	Handler.Run	94.74%	95.00%	+0.26%

[nghialv]

/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.

[nghialv]

/changelog

[pipecd-bot]

@nghialv: Changelog has been generated in response to this comment.

Details

Changelog since v0.11.0

Notable Changes

• Fix bug that all plan-preview commands that come after Piped has been running for 5 minutes are not handled (#2222)
• Allow embedding piped-key and ssh-key in Piped config file (#2216)
• Fix bug that SAVE button at admin account form is not enabled when using the same username (#2209)

Internal Changes

• Add metrics for plan-preview component (#2223)
• Add more logs to plan-preview handler (#2221)
• Fix validation bug on Piped config (#2219)
• Let Prometheus not scrape unneeded targets (#2213)
• Copy SSH key into ssh config dir to use instead of using the given one directly (#2214)
• Delete unused workflow (#2211)
• Update OWNERS_ALIASES (#2210)
• Update Grafana dashboards (#2207)
• Avoid panic in case of error gathering ops metrics (#2206)
• Enable ops metrics endpoint (#2203)
• Expose piped stat metrics at ops metrics endpoint (#2202)
• Explicitly use string as type for key in cache package (#2200)