[ledger] In FetchPartial benchmark, only fetch one (but bigger) object.

This allows "split_first" to be actually useful, as we record the first
call to FetchPartial on an object separately from the rest.

TEST=trace record
--spec-file=/pkgfs/packages/ledger_benchmarks/0/data/fetch_partial_big_entry.tspec

Change-Id: Iac493d77f00515f7d01fd695917902153acfc450
diff --git a/bin/ledger/tests/benchmark/fetch/fetch_partial_big_entry.tspec b/bin/ledger/tests/benchmark/fetch/fetch_partial_big_entry.tspec
index 33c4c8c..08a2dff 100644
--- a/bin/ledger/tests/benchmark/fetch/fetch_partial_big_entry.tspec
+++ b/bin/ledger/tests/benchmark/fetch/fetch_partial_big_entry.tspec
@@ -1,7 +1,7 @@
 {
   "test_suite_name": "fuchsia.ledger",
   "app": "fuchsia-pkg://fuchsia.com/ledger_benchmarks#meta/fetch.cmx",
-  "args": ["--entry-count=20", "--value-size=100000", "--part-size=25000"],
+  "args": ["--entry-count=1", "--value-size=10000000", "--part-size=50000"],
   "categories": ["benchmark", "ledger"],
   "duration": 120,
   "measure": [
@@ -9,8 +9,7 @@
       "type": "duration",
       "event_name": "Fetch (cumulative)",
       "event_category": "benchmark",
-      "expected_sample_count": 20,
-      "split_first": true
+      "expected_sample_count": 1
     },
     {
       "type": "duration",