opentelemetrytracer: allow configuring flush_interval_ms and min_flush_spans with tracer config

[joaopgrassi]

Title: allow configuring flush_interval_ms and min_flush_spans with tracer config

Description:
The OpenTelemetry tracer has two runtime settings that control the behavior of when spans are exported. One is flush_interval_ms which controls the timeout for a export to happen and the other is min_flush_spans which controls the size of the batch to trigger also a export.

Both of these today need to be enabled via the runtime config. I may be lacking context but when using Istio, to be able to change these I would need to deploy a ConfigMap and add a annotation to all my pods in order for Istio to know that it needs to "merge" this config when injecting the sidecar proxies.

I'm not sure if deploying configmaps is a common thing and users are used to it, but as a OpenTelemetry contributor/user, such configurations are part of the "SDK" configuration. For ex, the batch processor has similar parameters. Similarly, the OTel collector also has the same.

So I thought Envoy could also expose these two properties as part of the "normal" OTel tracer:

provider:
  name: envoy.tracers.opentelemetry
  typed_config:
    "@type": type.googleapis.com/envoy.config.trace.v3.OpenTelemetryConfig
    service_name: envoy-HTTP-exporter
    [other config]
    flush_interval_ms: 6000 <<<<<< NEW
    min_flush_spans: 512    <<<<<< NEW

[optional Relevant Links:]
#35997 (comment)

[kyessenov]

CC @AlexanderEllis