Added support to read diagnostics config from the path defined in environment variable

[Yun-Ting]

Fix Issue #2724.

Changes

Design assumption:
Function Apps environment will set the value of this environment variable: APPLICATIONINSIGHTS_LOG_DIAGNOSTICS to be a directory that

1. Users can create/delete diagnostics file and,
2. SDK has the privilege to write to
   at host initialization.

The SDK reads the value of APPLICATIONINSIGHTS_LOG_DIAGNOSTICS, and parse settings of ApplicationInsightsDiagnostics.json file and write diagnostics log to the directory.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

[xiang17]

LGTM

[xiang17]

remove the blank space before else if.

[Yun-Ting]

do we want to have a blank line before comment?

[cijothomas]

Function Apps environment will set the value of this environment variable: APPLICATIONINSIGHTS_LOG_DIAGNOSTICS to be a directory that

Users can create/delete diagnostics file and,
SDK has the privilege to write to
at host initialization.
The SDK reads the value of APPLICATIONINSIGHTS_LOG_DIAGNOSTICS, and parse settings of ApplicationInsightsDiagnostics.json file and write diagnostics log to the directory.

Bit confused with this description.
The following is my understanding:
The SDK needs read permission for the directory specified in APPLICATIONINSIGHTS_LOG_DIAGNOSTICS.
The SDK needs write permission for the directory specified in ApplicationInsightsDiagnostics.json, it could be different directory than specified in APPLICATIONINSIGHTS_LOG_DIAGNOSTICs.

[Yun-Ting]

it could be different directory than specified in APPLICATIONINSIGHTS_LOG_DIAGNOSTICs.

@cijothomas :
Not really. Both the ApplicationInsightsDiagnostics.json and the log file, for example: 20010101-120000.foobar.exe.12345.log will be placed in the directory that was set as the value of the environment variable of APPLICATIONINSIGHTS_LOG_DIAGNOSTICS.

See:

ApplicationInsights-dotnet/BASE/src/Microsoft.ApplicationInsights/Extensibility/Implementation/Tracing/SelfDiagnostics/SelfDiagnosticsConfigParser.cs

Line 50 in 6525f04

logDirectory = logDiagnosticsPath;

[TimothyMothra]

@Yun-Ting please add an entry in the changelog.