Allow Adding Custom Span Attributes Before `startSpan()`

#### Describe the feature

In order to use OpenTelemetry Head Sampling based on specific data information, I can extract the data upfront and set into Span Attribute before `startSpan()` for Head Sampling working with my custom Attribute.

```
tracer.spanBuilder("my.class")
.setAttribute("custom", "123")
.startSpan();
```

But in Vertx OpenTelemetry Tracing, there is no way for me to configure or intercept how to insert my own Attributes into Span before startSpan().

Using `Span.current()` is not working for me as the Span only initialize in Vertx low level implementation when request is received.

For what I observe, the TagExtractor used for Attributes building and it's fixed from Vert.x core.
https://github.com/eclipse-vertx/vertx-tracing/blob/master/vertx-opentelemetry/src/main/java/io/vertx/tracing/opentelemetry/OpenTelemetryTracer.java#L148-L154
https://github.com/eclipse-vertx/vert.x/blob/3e9d5d151e0bb91825fb214d94e72f645c0d6e60/src/main/java/io/vertx/core/http/impl/HttpUtils.java#L69-L163

Will be greats if we can have some control the Span Attributes before it start, so Head Sampling can be much useful.

#### Use cases

We have the data before it sent out as request and wanted to use it to perform OpenTelemetry Trace filtering.

#### Contribution

I may try to contribute but need some insight which is best approach to implement this.