disable `@opentelemetry/api` instrumentation (for OTel Metrics API support) unless `opentelemetryBridgeEnabled=true`

[trentm]

Currently opentelemetryBridgeEnabled only controls whether the OTel Tracing API support is enabled. IOW, the "bridge" is somewhat confusingly sometimes just referring to the tracing API and not the metrics API. I think this is historical.

IMO, it is surprising that the wrapping of @opentelemetry/api's getMeterProvider can be active even if opentelemetryBridgeEnabled is false.

(Anecdotally this surprise was noticed in a crazy case where this led to a crash because the application in question was (a) using metrics.getMeter(...) from @opentelemetry/api, (b) without having registered a MeterProvider (which is the case where elastic-apm-node's wrapping with do work), (c) without the intention of using elastic-apm-node's bridge, (d) with elastic-apm-node enabled, and critically (e) with elastic-apm-node's dependency on @opentelemetry/sdk-node having been forced to the current v2 rather than the v1 is depends on.)

While this could be a breaking change, the OTel API support is experimental, so we could do so in a minor.