Fix upload/download stats to use wall-clock duration

[amit-writes-code]

Use edge detection (Start/Stop) to bracket visitProtoPayloads rather than
summing individual PayloadBatchCompleted durations

Fixes #2378

What was changed

internal/internal_task_pollers.go

• Replaced totalDuration time.Duration field on workflowTaskStorageMetrics
  with startTime and endTime time.Time fields.
• Added Start() — records wall-clock start immediately before
  visitProtoPayloads is called (rising edge).
• Added Stop() — records wall-clock end immediately after
  visitProtoPayloads returns (falling edge), before any subsequent
  workflow execution or network calls.
• Added WallClockDuration() — returns endTime.Sub(startTime), a fixed
  interval between two captured timestamps. Replaces totalDuration at
  both log sites (tagPayloadDownloadDuration, tagPayloadUploadDuration).
• PayloadBatchCompleted no longer accumulates duration — it only tracks
  payloadCount, totalSize, and driverNames.

internal/internal_task_pollers_test.go

• TestWorkflowTaskStorageMetrics_WallClockDuration — verifies the
  Start/Stop/WallClockDuration lifecycle: returns 0 before Start(),
  returns 0 before Stop(), returns a positive duration after Stop(),
  does not shrink on a second Stop() call.
• TestWorkflowTaskStorageMetrics_PayloadBatchCompleted — simulates two
  concurrent callbacks and asserts that payloadCount and totalSize
  accumulate correctly, driver names are collected from both callbacks,
  and wall-clock duration is less than the sum of individual durations
  (proving edge detection rather than summation).

Why?

PayloadBatchCompleted was called once per Visit invocation with a
duration argument representing the wall-clock time for that Visit.
When payloadVisitorConcurrency > 1, multiple Visit calls run
concurrently. Summing their individual durations overcounts proportionally
to the number of concurrent visits:

Visit 1: t=0ms  → t=100ms   duration=100ms
Visit 2: t=10ms → t=120ms   duration=110ms   (runs concurrently with Visit 1)

Old: totalDuration = 100 + 110 = 210ms  ← wrong, they overlapped
New: Stop() − Start() = 120ms − 0ms = 120ms  ← correct wall-clock

Checklist

1. Closes [Bug] External storage transfer stats reports summation of durations instead of wall-clock duration #2378
2. How was this tested:
   • TestWorkflowTaskStorageMetrics_WallClockDuration — unit test for
     the Start/Stop/WallClockDuration lifecycle.
   • TestWorkflowTaskStorageMetrics_PayloadBatchCompleted — concurrent
     callbacks assert wall-clock < sum of individual durations.
   • Run: go test ./internal/ -run "TestWorkflowTaskStorageMetrics" -v
3. Any docs updates needed? No — internal observability change only, no
   public API or user-visible behavior change.

[amit-writes-code]

@jmaeagle99 would love to know if there is any scope of optimzations here, in terms of accuracy