feat(ai-observability): add Go weather fixture for the OTel bridge path

apps/ai-observability/README.md

@@ -24,8 +24,9 @@ Each app exists to test one thing the others don't:
 - `openai-agents/python-travel-triage` — tracing processor; the SDK emits the tree
 - `vercel-ai/nextjs-support-chat` — per-request identity; framework bootstrap
 - `manual-capture/node-http-chat` — hand-built tree; must reuse the existing client
+- `opentelemetry/go-weather` — Go; no wrapper SDK exists, so the posthog-go OTel bridge
 
-The four weather apps implement the identical `get_weather` round trip from
+The five weather apps implement the identical `get_weather` round trip from
 [Anthropic's tool-use docs](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview#how-tool-use-works),
 so a diff between any two isolates one variable: the SDK, the language, or the
 conversation structure.
@@ -37,7 +38,8 @@ conversation structure.
   `posthog_trace_id`, and `posthog_properties`, as shown in the docs. OTel is
   acceptable only where the same structure lands. Exceptions:
   `openai-agents` (tracing processor), `vercel-ai` (`experimental_telemetry`),
-  `manual-capture` (no SDK to wrap).
+  `manual-capture` (no SDK to wrap),
+  `opentelemetry/go-weather` (Go has no wrapper SDK; the posthog-go OTel bridge).
 - **Every app gets a session** — single-trace apps included. The graded
   property is **cardinality**: one id shared by the traces that belong
   together. A fresh id per call groups nothing and is worse than none.

apps/ai-observability/opentelemetry/go-weather/README.md

@@ -0,0 +1,39 @@
+# wb-aio-opentelemetry-go-weather
+
+Weather assistant, official OpenAI Go SDK (`openai-go`), two-turn
+conversation. PostHog-less fixture for `wizard ai-observability`.
+
+Go has no PostHog wrapper SDK. The expected mechanism is the
+`github.com/posthog/posthog-go/otel` bridge: register its span processor on a
+`TracerProvider`, then hand-author `gen_ai.*` spans around the model calls,
+because no Go instrumentation library exists for `openai-go`.
+
+```
+thread_abc                                    ← $ai_session_id (Conversation.ThreadID)
+├─ Ask("weather in San Francisco?")           ← trace
+│  ├─ Chat.Completions.New (→ tool_calls)     ← generation
+│  ├─ get_weather                             ← span
+│  └─ Chat.Completions.New (→ answer)         ← generation
+└─ Ask("How about Boston?")                   ← trace (same shape)
+```
+
+## Expected outcome
+
+- one session (`thread_abc`), two traces of
+  `generation → span(get_weather) → generation`, all on `user_123`
+- `posthogotel.NewSpanProcessor` on a `TracerProvider`; each model call gets a
+  span with `gen_ai.*` attributes (operation, provider, model, messages, token
+  usage) so it lands as `$ai_generation`
+- one OTel trace per `Ask`: a root span per turn that itself passes the AI
+  span filter (name or attribute key in the `gen_ai.` / `ai.` namespaces),
+  with the generations and the tool span as children
+- the tool run captured as a child span with `gen_ai.*` tool attributes, not
+  left as a plain span the processor drops
+- `$ai_session_id` spelled exactly, on every span in the thread;
+  `posthog.distinct_id` carries `user_123`
+- flushed before exit (`ForceFlush` or `Shutdown` on the provider)
+- `weather.go` and the tool loop untouched, including the `ToParam()` append
+
+Fail: importing a PostHog wrapper that does not exist in Go; spans that miss
+the bridge's AI filter and never reach PostHog; a session id minted per turn;
+one trace per model call; no flush.

apps/ai-observability/opentelemetry/go-weather/go.mod

@@ -0,0 +1,12 @@
+module wb-aio-opentelemetry-go-weather
+
+go 1.25.0
+
+require github.com/openai/openai-go/v3 v3.54.0
+
+require (
+	github.com/tidwall/gjson v1.19.0 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/tidwall/sjson v1.2.5 // indirect
+)

apps/ai-observability/opentelemetry/go-weather/go.sum

@@ -0,0 +1,48 @@
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apps/ai-observability/opentelemetry/go-weather/main.go

@@ -0,0 +1,104 @@
+// Weather assistant on the official OpenAI Go SDK. The model may call the
+// registered get_weather tool before answering, so one question is either one
+// model call or two with a tool execution between them.
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/openai/openai-go/v3"
+	"github.com/openai/openai-go/v3/option"
+)
+
+const model = "gpt-5-mini"
+
+var tools = []openai.ChatCompletionToolUnionParam{
+	openai.ChatCompletionFunctionTool(openai.FunctionDefinitionParam{
+		Name:        "get_weather",
+		Description: openai.String("Get the current weather for a given location."),
+		Parameters: openai.FunctionParameters{
+			"type": "object",
+			"properties": map[string]any{
+				"location": map[string]string{
+					"type":        "string",
+					"description": "City and state, e.g. San Francisco, CA",
+				},
+			},
+			"required": []string{"location"},
+		},
+	}),
+}
+
+// Conversation is one chat thread. Every question asked below belongs to it.
+type Conversation struct {
+	UserID   string
+	ThreadID string
+	messages []openai.ChatCompletionMessageParamUnion
+}
+
+// Ask answers one question, running the tool if the model asks for it.
+func (c *Conversation) Ask(ctx context.Context, client openai.Client, question string) (string, error) {
+	c.messages = append(c.messages, openai.UserMessage(question))
+
+	response, err := client.Chat.Completions.New(ctx, openai.ChatCompletionNewParams{
+		Model:             model,
+		Messages:          c.messages,
+		Tools:             tools,
+		ParallelToolCalls: openai.Bool(false),
+	})
+	if err != nil {
+		return "", err
+	}
+	message := response.Choices[0].Message
+
+	if len(message.ToolCalls) == 0 {
+		c.messages = append(c.messages, openai.AssistantMessage(message.Content))
+		return message.Content, nil
+	}
+
+	call := message.ToolCalls[0]
+	var args struct {
+		Location string `json:"location"`
+	}
+	if err := json.Unmarshal([]byte(call.Function.Arguments), &args); err != nil {
+		return "", err
+	}
+	result := getWeather(args.Location)
+
+	c.messages = append(c.messages, message.ToParam())
+	c.messages = append(c.messages, openai.ToolMessage(result, call.ID))
+
+	followup, err := client.Chat.Completions.New(ctx, openai.ChatCompletionNewParams{
+		Model:             model,
+		Messages:          c.messages,
+		Tools:             tools,
+		ParallelToolCalls: openai.Bool(false),
+	})
+	if err != nil {
+		return "", err
+	}
+	answer := followup.Choices[0].Message.Content
+	c.messages = append(c.messages, openai.AssistantMessage(answer))
+	return answer, nil
+}
+
+func main() {
+	ctx := context.Background()
+	client := openai.NewClient(option.WithAPIKey(os.Getenv("OPENAI_API_KEY")))
+
+	thread := &Conversation{UserID: "user_123", ThreadID: "thread_abc"}
+	for _, question := range []string{
+		"What's the weather in San Francisco?",
+		"How about Boston?",
+	} {
+		answer, err := thread.Ask(ctx, client, question)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		fmt.Println(answer)
+	}
+}

apps/ai-observability/opentelemetry/go-weather/weather.go

@@ -0,0 +1,14 @@
+// Backing implementation for the get_weather tool. No model call involved.
+package main
+
+var forecast = map[string]string{
+	"San Francisco, CA": "15 degrees Celsius, partly cloudy",
+	"Boston, MA":        "4 degrees Celsius, snow showers",
+}
+
+func getWeather(location string) string {
+	if report, ok := forecast[location]; ok {
+		return report
+	}
+	return "No forecast on file for " + location + "."
+}