Add missing Meter Docs

[tarekgh]

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Issue Details

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Author:	tarekgh
Assignees:	tarekgh
Labels:	area-System.Diagnostics
Milestone:	-

[tarekgh]

CC @carlossanlop @gewarren

[learn-build-service-prod]

Learn Build status updates of commit 9d1a352:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics.Metrics/IMeterFactory.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument`1.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Meter.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterFactoryExtensions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterOptions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/ObservableInstrument`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[carlossanlop]

Reviewed 75%. Left some suggestions.

[carlossanlop]

Rest of them.

[learn-build-service-prod]

Learn Build status updates of commit e023381:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
	❌Error		Details
xml/System.Diagnostics.Metrics/Instrument`1.xml	❌Error		Details

• Line 0, Column 0: [Error: ECMA2Yaml_File_LoadFailed] Failed to load 1 files, aborting...

xml/System.Diagnostics.Metrics/Instrument`1.xml

• Line 0, Column 0: [Error: ECMA2Yaml_InternalError]

Intenal Several Error: System.Xml.XmlException: 'T' is an unexpected token. Expecting whitespace. Line 143, position 52.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.Throw(String res, String arg)
   at System.Xml.XmlTextReaderImpl.ThrowExpectingWhitespace(Int32 pos)
   at System.Xml.XmlTextReaderImpl.ParseAttributes()
   at System.Xml.XmlTextReaderImpl.ParseElement()
   at System.Xml.XmlTextReaderImpl.ParseElementContent()
   at System.Xml.Linq.XContainer.ReadContentFrom(XmlReader r)
   at System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options)
   at System.Xml.Linq.XDocument.Parse(String text, LoadOptions options)
   at ECMA2Yaml.ECMALoader.LoadType(FileItem typeFile)
   at ECMA2Yaml.ECMALoader.LoadTypes(String basePath, Namespace ns)

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit ad32746:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics.Metrics/IMeterFactory.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument`1.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Meter.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterFactoryExtensions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterOptions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/ObservableInstrument`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit ae90263:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics.Metrics/IMeterFactory.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument`1.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Meter.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterFactoryExtensions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterOptions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/ObservableInstrument`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit b7c8bdf:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics.Metrics/IMeterFactory.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Instrument`1.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/Meter.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterFactoryExtensions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/MeterOptions.xml	✅Succeeded	View
xml/System.Diagnostics.Metrics/ObservableInstrument`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel