Fix metrics-collector panic on invalid regular expression filters

[AdeshDeshmukh]

What this PR does / why we need it:

Fixes a crash in the file-metricscollector sidecar when a user-provided metrics filter is not a valid regular expression.

GetFilterRegexpList discarded the error from regexp.Compile, so an invalid filter produced a nil *regexp.Regexp entry in the returned list. The first pattern match against a log line (FindAllStringSubmatch) then dereferenced the nil regexp and panicked the metrics-collector container — either in parseLogsInTextFormat or in the watch loop in main.go — so no metrics were ever reported for the Trial, with no hint that the user's filter was the problem.

With this change:

• The compile error is handled and the collector fails fast at startup with a descriptive message identifying the invalid filter (invalid metric filter "...": ...), consistent with the existing error handling in watchMetricsFile.
• GetFilterRegexpList now returns an error, propagated through parseLogsInTextFormat / CollectObservationLog.
• Filters are compiled once at startup instead of on every log line in the tail loop.

Behavior change note: previously an invalid filter silently matched nothing; now the sidecar exits immediately with a clear error. This is intentional — silent failure is what left Trials stuck without metrics.

Tests: added TestGetFilterRegexpList covering invalid filter (returns descriptive error, no panic), empty filters (default filter fallback), and valid multi-filter compilation. Existing TestCollectObservationLog cases pass unchanged.

Which issue(s) this PR fixes:
Fixes #2707

Checklist:

• Docs included if any changes are user facing

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[vjkumar2756]

@AdeshDeshmukh
This is a great fix that addresses both a stability issue (preventing potential runtime panics) and a performance bottleneck in the file metrics collector.

some Suggestions:

1. Duplicate Error Message (Bug): In main.go, klog.Fatalf("Invalid metric filter: %v", err) duplicates the error prefix because GetFilterRegexpList already returns invalid metric filter %q. Change to klog.Fatalf("Failed to compile metric filters: %v", err).

2. Sub-optimal Memory Allocation: In GetFilterRegexpList(), make([]*regexp.Regexp, 0, len(filters)) is called before handling len(filters) == 0, allocating 0 capacity. Check/set default filters before allocating regexpList.

3. Typo: In main.go comment, fix "regural expressions" -> "regular expressions".