[Metrics SDK] Cleanup of old metrics implementation

[lalitb]

Changes required

• delete files under api//_metrics, sdk//_metrics directory.
• remove ENABLE_METRICS_PREVIEW macro check in new metrics implementation.
• remove old Prometheus exporter, old metrics ostream exporter.
• remove examples/metrics_simple/main.cc
• fix cmake/bazel scripts.
• cleanup old metrics tests from CI

[lalitb]

Already implemented.