[instrumentation] - Suppress tracing using environment variables

[maorleger]

Packages impacted by this PR

@azure/opentelemetry-instrumentation-azure-sdk

Issues associated with this PR

Resolves #17969

Describe the problem that is addressed by this PR

In order to mitigate duplication of AZ SDK HTTP spans with auto-instrumentation
HTTP spans, the tracing feature crew would like to add the
AZURE_HTTP_TRACING_DISABLED environment variable to disable any dowstream
HTTP spans from being created.

In addition, I realized that when migrating off-of createSpan we did not
migrate the logic to suppress all tracing using AZURE_TRACING_DISABLED
environment variable.

This PR fixes both of these issues.

What are the possible designs available to address the problem? If there are more than one possible design, why was the one in this PR chosen?

The alternatives really were:

• Add this logic in core-tracing for global suppression
• Add this logic in tracingPolicy for HTTP span suppression

Since we use a no-op instrumenter by default there's no reason to add additional
logic to core-tracing. Further, using it in the OTel package allows us to take
advantage of OTel provided APIs which will be honored downstream as well.

Quick summary of requirements here:

• When AZURE_TRACING_DISABLED is set, we want to suppress our Azure SDK spans from being recorded
• When AZURE_HTTP_TRACING_DISABLED is set, we want to suppress any children of our SDK's HTTP spans from being recorded, this avoids the HTTP span duplication problem while OTel works out a solution on their end
• Finally, if both are set, we suppress our Azure SDK spans from being recorded, but not suppress any children of our HTTP spans (since there's no duplication)

Are there test cases added in this PR? (If not, why?)

Yes

Provide a list of related PRs (if any)

Command used to generate this PR: (Applicable only to SDK release request PRs)

Checklists

• Added impacted package name to the issue description.
• Does this PR need any fixes in the SDK Generator?** (If so, create an Issue in the Autorest/typescript repository and link it here.)
• Added a changelog (if necessary).

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[lmolkova]

Left a couple of comments on reading env vars on the hot path. Caching them will probably cause some complications for tests, but seems important. Otherwise, looks great!

[xirzec]

A few minor comments