Azure Monitor Exporter API changes based on arch board review

[srnagar]

Based on the arch board feedback for Azure Monitor exporter, update the APIs to support the following:

• Include an option to configure the service API version when constructing the exporter instance
• Provide a convenience method on the exporter to get the endpoint
• Consider adding APIs for configuring endpoint and instrumentationKey separately. Continue to support connectionString which contains both endpoint and instrumentationKey.

[joshfree]

/cc @rakshith91

[rakshith91]

Agree with the changes - @lzchen let me know if you need any help on this one