Bug: TelemetryClient caching ignores package_name parameter

[godobyte]

Describe the bug

The get_telemetry_client() function uses a global cache that ignores the package_name parameter when returning an existing client. This causes incorrect service attribution in telemetry events when multiple components request clients with different package names.

For context - #857

Root Cause
• Single global __cached_telemetry_client variable shared across all package names
• Cache key doesn't include package_name, only checks if any client exists
• Class-level _system_metadata dictionary is shared across all instances, causing service name overwrites

Expected Behaviour

• get_telemetry_client("deadline-cloud-worker-agent", version) should return a client with service="deadline-cloud-worker-agent"
• get_telemetry_client("deadline-cloud-library", version) should return a client with service="deadline-cloud-library"
• Each unique package name should have its own cached client instance

Current Behaviour

• The first call to get_telemetry_client() creates a client and caches it globally
• Subsequent calls with different package_name parameters return the same cached client
• All telemetry events use the service name from the first client created, regardless of which component emits them

Reproduction Steps

Example:

1. Job attachments calls get_telemetry_client("deadline-cloud-library", version) → creates client with service="deadline-cloud-library"
2. Worker agent calls get_telemetry_client("deadline-cloud-worker-agent", version) → returns same cached client with service="deadline-cloud-library"
3. Worker agent telemetry events incorrectly show service="deadline-cloud-library"

Environment

At minimum:

1. Operating system (e.g. Windows Server 2022; Amazon Linux 2023; etc.)
2. Version of this package

Please share other details about your environment that you think might be relevant to reproducing the bug.

[godobyte]

This is not the intended repository, please refer to aws-deadline/deadline-cloud#959.