otelhttp: allow setting the start timestamp using go Context

[dashpole]

Problem Statement

Follow-up to open-telemetry/opentelemetry-go#4993

Context: kubernetes/kubernetes#103186

I would like otelhttp to allow me to pass the start time using the go context.Context. This lets me move my instrumentation after my auth filter in my server handler chain while preserving the start time of the request.

Proposed Solution

Similar to the Labeler, allow adding a start timestamp to a context in otelhttp. This timestamp will be used by both otelhttp metric and trace instrumentation as the start timestamp.

Alternative: Context-based StartSpanOptions

In the trace API package, add a SpanStartContext which is modeled after the current Labeler, but stores a list of SpanStartOptions, and has Add and Get options for the StartSpanOptions.

In the trace SDK, in Start, it would fetch options from the SpanStartContext, and append them to the explicitly passed options.

This would allow users to override the StartSpanOptions for any instrumentation library, including adding attributes or links, or setting the span start time without any modifications to the instrumentation library itself.

However, this would not work very well for metrics. First, there is no "start timestamp" option for metrics (nor should there be). The start timestamp is an artifact of the otelhttp instrumentation library, not of the metrics API. Second, while tracing instrumentation generally only creates one span, metric instrumentation generally makes multiple Record calls. The rule of "these options apply to the span created" doesn't work well for metrics, because we probably want them to apply to all metrics in the instrumentation library.