Support change the context prefix and suffix format of KeyValueFormatter

[kuraun]

• Add MetadataKey "prefix" and "suffix" of LogContext
• Modify to generate LogMessageFormatter using MetaData

Related: #275

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[kuraun]

@googlebot I signed it!

[sebastienvermeille]

Would be a great improvement do we have any news regarding it ?

Thank you

[hagbard]

The right way to fix this is to open-source the customizable message formatter we currently use internally. This allows more customization than just changing the suffix/prefix.