[internal/datadog] Fix Datarace in metrics client

[dineshg13]

Description:

Fixes the data race.

fatal error: concurrent map read and map write

goroutine 72665 [running]:
github.com/open-telemetry/opentelemetry-collector-contrib/internal/datadog.(*metricsClient).Gauge.func1({0x7b895a0?, 0xc07c930790?}, {0x940dac8, 0xc1996ce180})
	github.com/open-telemetry/opentelemetry-collector-contrib/internal/[email protected]/metrics_client.go:49 +0x85
go.opentelemetry.io/otel/sdk/metric.(*meter).float64ObservableInstrument.func1.1({0x9443f08, 0xef10b00})
	go.opentelemetry.io/otel/sdk/[email protected]/meter.go:286 +0x55
go.opentelemetry.io/otel/sdk/metric.(*pipeline).produce(0xc002852bd0, {0x9443f08, 0xef10b00}, 0xc108dd46c0)
	go.opentelemetry.io/otel/sdk/[email protected]/pipeline.go:122 +0x168
go.opentelemetry.io/otel/sdk/metric.(*ManualReader).Collect(0xc0030d2aa0, {0x9443f08, 0xef10b00}, 0xc108dd46c0)
	go.opentelemetry.io/otel/sdk/[email protected]/manual_reader.go:123 +0xe2
go.opentelemetry.io/otel/exporters/prometheus.(*collector).Collect(0xc0030e4aa0, 0xc10a0083c0)
	go.opentelemetry.io/otel/exporters/[email protected]/exporter.go:158 +0x72
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
	github.com/prometheus/[email protected]/prometheus/registry.go:457 +0xe5
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather in goroutine 5

Link to tracking Issue:

Testing:

Documentation:

[mx-psi]

You will need to rebase this one after #31972 is merged

[songy23]

Does this need a changelog? Seems users can run into this bug

[dineshg13]

@songy23 I would prefer not to add changelog for a data race bug.

[mx-psi]

I agree with Yang that this would need a changelog, I mistakenly assumed this did not have user-facing implications