update SnippetGenerator to add language hint

[christothes]

This change adds the language="csharp" property to the <code> xml tag and also makes it possible to error on unused snippets.

For unused snippet validation, there is a new switch added to Update-Snippets.ps1 called -StrictMode. Usage is as follows:

The default behavior is to print the names of unused snippets, but not to thow:

 .\eng\scripts\Update-Snippets.ps1 tables
Discovered snippets:
<...>

Not all snippets were used.
Snippet:TablesSample1CreateTableAsync
Snippet:TablesSample1DeleteTableAsync
<...>
Snippet:TablesSample5UpsertWithReplace
Snippet:TablesSample5UpdateEntity
SnippetGenerator completed in 00:00:00.5049468

With the StrictMode switch, it looks like this:

 .\eng\scripts\Update-Snippets.ps1 tables -StrictMode
Discovered snippets:
<...>

System.InvalidOperationException: Not all snippets were used.
Snippet:TablesSample1CreateTableAsync
Snippet:TablesSample1DeleteTableAsync
<...>
Snippet:TablesSample5UpsertWithReplace
Snippet:TablesSample5UpdateEntity
SnippetGenerator completed in 00:00:00.5049468

created the following issues to track remediation of unused snippets https://github.com/Azure/azure-sdk-for-net/search?q=Remove+unused+snippets&type=issues

[heaths]

General and code-owned files LGTM. Curious, though: is this a convention for Microsoft Docs? According to https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/xmldoc/code the language attribute isn't defined. Based on https://docs.microsoft.com/en-us/contribute/code-in-docs it seems that might be the case since there is similar syntax for including snippets from other files, but I wanted to double check.

[heaths]

Nit: should use Async suffix.

[pakrym]

Should we start erroring out for unused snippets? It would prevent us from accidentally breaking the generator and people from using the wrong snippet (like I did very recently).

[christothes]

Should we start erroring out for unused snippets? It would prevent us from accidentally breaking the generator and people from using the wrong snippet (like I did very recently).

What should we do with the unused snippets? Ran locally with the changes and got this:

System.InvalidOperationException: Not all snippets were used.
Snippet:IotHubCreateDeviceIdentities
Snippet:IotHubUpdateDeviceIdentities
Snippet:IotHubUpdateDeviceTwins
Snippet:IotHubDeleteDeviceIdentities
Snippet:IotHubGetDeviceIdentities
Snippet:IotHubGetDeviceTwins
Snippet:IotHubCreateModuleIdentities
Snippet:IotHubUpdateModuleIdentities
Snippet:IotHubUpdateModuleTwins
Snippet:IotHubDeleteModuleIdentities
Snippet:IotHubGetModuleIdentities
Snippet:IotHubGetModuleTwins
Snippet:IotHubCreateConfiguration
Snippet:IotHubGetConfiguration
Snippet:IotHubUpdateConfiguration
Snippet:IotHubDeleteConfiguration
Snippet:IotHubImportJob
Snippet:IotHubExportJob
Snippet:GetImportExportJob
Snippet:IotInvokeMethodOnModule
Snippet:IotInvokeMethodOnDevice
Snippet:IotHubCreateModuleIdentity
Snippet:IotHubGetModuleIdentities
Snippet:IotHubGetModuleIdentity
Snippet:IotHubUpdateModuleIdentity
Snippet:IotHubGetModuleTwin
Snippet:IotHubUpdateModuleTwin
Snippet:IotHubDeleteModuleIdentity
Snippet:IotHubQueryDeviceTwins
Snippet:IotHubQueryModuleTwins
Snippet:IotHubGetDeviceStatistics
Snippet:IotHubGetServiceStatistics

[pakrym]

What should we do with the unused snippets? Ran locally with the changes and got this:

Delete them or comment out the #region part. They are still in source control, we just need to make sure the team is aware