fix(metricscollector): parse fractional UNIX timestamps in JSON logs

[immanuwell]

What this PR does / why we need it

file-metricscollector supports JSON logs, but numeric UNIX timestamps with fractions were parsed wrong. The fractional part was treated like raw nanoseconds, so 1638422847.28721 became 2021-12-02T05:27:27.000028721Z instead of 2021-12-02T05:27:27.28721Z.

This patch keeps the fractional seconds when building the RFC3339 timestamp. String timestamps and integer UNIX timestamps stay the same. This is a small fix, but it is pretty easy to hit on the JSON log path.

Related to #944
Follow-up to #1765

How to reproduce

1. Run go test ./pkg/metricscollector/v1beta1/file-metricscollector -count=1
2. Before this patch, the JSON fixture in TestCollectObservationLog stored 1638422847.28721 as 2021-12-02T05:27:27.000028721Z
3. With this patch, it stores 2021-12-02T05:27:27.28721Z

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[immanuwell]

@anencore94 @Electronic-Waste @gaocegege please take a look