Add http.user_agent attribute before startSpan so it's available for sampling

[trask]

We had an Application Insights user try to configure sampling based on http.user_agent but was unsuccessful since that attribute is not added prior to starting the span (microsoft/ApplicationInsights-Java#2083 (comment))

azure-sdk-for-java/sdk/core/azure-core-tracing-opentelemetry/src/main/java/com/azure/core/tracing/opentelemetry/OpenTelemetryHttpPolicy.java

Lines 124 to 127 in 31b8373

	private static void addPostSamplingAttributes(Span span, HttpRequest request,
	HttpPipelineCallContext context) {
	putAttributeIfNotEmptyOrNull(span, HTTP_USER_AGENT,
	request.getHeaders().getValue("User-Agent"));

[lmolkova]

@trask should we mark user-agent as sampling-relevant in the HTTP semconv?

[lmolkova]

Closing this since user_agent.original became opt-in and Azure SDKs no longer report it