Remove context from metrics force_flush

[cijothomas]

More like a follow up clean up from : #1076

[codecov]

Codecov Report

Patch has no changes to coverable lines.

Files Changed	Coverage
opentelemetry-prometheus/src/lib.rs	0.0%
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%
opentelemetry-sdk/src/metrics/meter_provider.rs	0.0%
opentelemetry-sdk/src/metrics/periodic_reader.rs	0.0%
opentelemetry-sdk/src/metrics/pipeline.rs	0.0%
opentelemetry-sdk/src/metrics/reader.rs	ø
...etry-sdk/src/testing/metrics/in_memory_exporter.rs	ø
...telemetry-sdk/src/testing/metrics/metric_reader.rs	0.0%

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