App Configuration provider: add telemetry schme

[ivywei0125]

Description

Telemetry schema

{
    "telemetry": {
        "enabled": true,
        "metadata": {
            "key": "value",
            "key2": "value2"
        }
    }
}

Metadata is represented by a dictionary of string keys and string values. When telemetry is enabled, need to add FeatureFlagId, FeatureFlagReference, and ETag. These 3 properties are agreed upon with Split.io used to trace feature flag.

• FeatureFlagId – an ID for each feature flag based on the key and label that is then hashed with SHA 256 and encoded to base64 URL.
  b64_url(SHA256(utf8_bytes(“${featureFlag.key}\n${featureFlag.label}”))).Replace('+', '-').Replace('/', '_').TrimEnd('=')
• FeatureFlagReference – a URL used to trace back from a feature flag to a specific app configuration store it came from.
  ${endpoint}kv/${featureFlag.key}?label=${featureFlag.label}
• ETag – The ETag returned as part of the configuration setting for a feature flag.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[mrm9084]

NIT: We have a TestConstants class with stuff like this in it.