Add benchmark thresholds for nightly (#1743)

Motivation:

The benchmarks are run on nightly builds but there are no thresholds.
This is causing nightly CI to fail as the benchmark package is now more
stringent about missing thresholds.

Modifications:

- Add thresholds for nightly

Result:

CI passes

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Add_binary.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 11,
+  "memoryLeaked" : 0,
+  "releaseCount" : 3012,
+  "retainCount" : 2000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Add_string.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 11,
+  "memoryLeaked" : 0,
+  "releaseCount" : 4012,
+  "retainCount" : 2000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Iterate_all_values.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 0,
+  "memoryLeaked" : 0,
+  "releaseCount" : 3001,
+  "retainCount" : 1000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_binary_values_stored.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 0,
+  "memoryLeaked" : 0,
+  "releaseCount" : 3001,
+  "retainCount" : 1000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_strings_stored.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 2000,
+  "memoryLeaked" : 0,
+  "releaseCount" : 6001,
+  "retainCount" : 2000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Iterate_string_values.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 0,
+  "memoryLeaked" : 0,
+  "releaseCount" : 3001,
+  "retainCount" : 1000,
+  "syscalls" : 0
+}

Performance/Benchmarks/Thresholds/main/GRPCSwiftBenchmark.Metadata_Remove_values_for_key.p90.json

@@ -0,0 +1,7 @@
+{
+  "mallocCountTotal" : 0,
+  "memoryLeaked" : 0,
+  "releaseCount" : 2002001,
+  "retainCount" : 1999000,
+  "syscalls" : 0
+}