adds metadata to sarif result

[dani-santos-code]

This is to add the Metadata field/info to the sarif. We currently display this info on the CLI. but it was missing on the sarif. Here's an example of what it will look like on Github UI:

[thepwagner]

I think err vs jsonError is worth fixing.

Other comments stem from me not fully understanding what Metadata will contain, and so getting paranoid about edge cases. Those are safe to ignore!

[thepwagner]

No blockers, just an optimization idea.

[thepwagner]

Nit: we don't use this value if len(formattedMap) > 0 below.

While it's pretty cheap to render {}, we could just skip that by moving the map construction and JSON serialization to be within the guard.
I checked and kubeaudit.Metadata is a typedef of map[string]string - could we just use that directly to skip all of the map construction?

var metadataTxt string
if len(result.Metadata) > 0 {
    metadata, jsonErr := json.Marshal(result.Metadata)
    ...
    metadataTxt = fmt.Sprintf("Metadata: %s\n", string(metadata))
}

Nobody will notice the like 8 bytes of memory that we save, but we will know. 😎

[dani-santos-code]

yeah! excellent point! applied here