[BUG] Custom context with JSON file reporter produces invalid JSON

[miki-shapr]

Describe the bug
Adding a custom context (--benchmark_context) and using the JSON reporter (--benchmark_out_format), the line preceding the custom context entry does not have a comma at the end resulting in an invalid JSON file.

System
Which OS, compiler, and compiler version are you using:

• OS: macOS
• Compiler and version: Apple clang version 12.0.5

To reproduce
Steps to reproduce the behavior:

1. sync to commit 80a6261
2. cmake -DCMAKE_BUILD_TYPE=Release ...
3. make
4. See error

Expected behavior
The line before the custom context entry should have a comman (,) at the end if there is a context entry.

Screenshots

Additional context
N/A