fix: avoid panic when a metric has only percentile statistics

[pujitha24]

Motivation:
createGetMetricStatisticsInput built a debug CLI-helper string by
unconditionally indexing statistics[0]. metric.Statistics entries are
split into classic statistics (Average, Sum, ...) and extendedStatistics
(percentiles such as p50/p90/p99, matched via promutil.Percentile).
When a metric's Statistics list contains only percentiles, the classic
statistics slice stays empty, so statistics[0] panics with
"index out of range [0] with length 0" and crashes the whole process.
This matches the report: a static-job metric (IngestionToInvocationStartLatency)
configured with statistics: [p50, p90, p99] triggers the panic on every
scrape.

Approach:
Guard the index access: only read statistics[0] for the debug log line
when len(statistics) > 0, otherwise use an empty string. The real
GetMetricStatisticsInput fields (Statistics, ExtendedStatistics) were
already built correctly beforehand and are unaffected; only the debug
log string construction was unsafe.

Validation:

• Added pkg/clients/cloudwatch/input_test.go with
  TestCreateGetMetricStatisticsInput_OnlyExtendedStatistics, which
  configures a metric with only percentile statistics (p50, p90, p99)
  and asserts createGetMetricStatisticsInput does not panic.
• Confirmed the test fails before the fix: git stash (reverting the
  fix) then go test ./pkg/clients/cloudwatch/... -run TestCreateGetMetricStatisticsInput_OnlyExtendedStatistics -v
  reproduces the exact panic
  ("runtime error: index out of range [0] with length 0" at
  input.go:75), matching the crash reported in the issue.
• git stash pop restored the fix; the same test then passes.
• go build ./... succeeds.
• go test ./... (full suite) passes.
• golangci-lint run ./pkg/clients/cloudwatch/... reports 0 issues.

Report: #1702
Signed-off-by: Pujitha Paladugu 10557236+pujitha24@users.noreply.github.com

Fixes #1702

[pujitha24]

Just checking in on this one — it's still green and rebased on main; happy to make any changes if something would help move review along.