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**Description**
Add return_code metric into result and revise unit tests.

Benchmarks: Code Revision - Revise metric name generation and default config for disk performance benchmark (#175)

**Description**
This commit revises disk performance benchmark, including:
1) Add missing benchmark name in default config;
2) Avoid using reserved character ':' in metric name.

**Description**
This commit adds readwrite I/O pattern for FIO benchmark. Read/write ratio is fixed at 4:1.

**Description**
Add disk performance microbenchmark.

**Major Revision**
- Add microbenchmark, example, test, config for disk performance.

**Minor Revision**
- Fix bugs in executor unit test related to default enabled tests.