chore: first metric (verb call counter)

[deniseli]

Instrument our first metric. Next: check if this metric pipes through to DD correctly.

Locally, the output via just otel-stream looks like:

ScopeMetrics #1
ScopeMetrics SchemaURL: 
InstrumentationScope ftl.call 

Metric #0
Descriptor:
     -> Name: requests
     -> Description: Count of FTL verb calls via the controller
     -> Unit: 
     -> DataType: Sum
     -> IsMonotonic: true
     -> AggregationTemporality: Cumulative

NumberDataPoints #0
Data point attributes:
     -> ftl.module.name: Str(time)
     -> ftl.verb.name: Str(time)
StartTimestamp: 2024-07-23 00:25:50.782759 +0000 UTC
Timestamp: 2024-07-23 00:25:59.069408 +0000 UTC
Value: 1

NumberDataPoints #1
Data point attributes:
     -> ftl.module.name: Str(echo)
     -> ftl.verb.name: Str(echo)
StartTimestamp: 2024-07-23 00:25:50.782759 +0000 UTC
Timestamp: 2024-07-23 00:25:59.069408 +0000 UTC
Value: 1
	{"kind": "exporter", "data_type": "metrics", "name": "debug"}
2024-07-23T00:26:00.745Z	info	MetricsExporter	{"kind": "exporter", "data_type": "metrics", "name": "debug", "resource metrics": 1, "metrics": 10, "data points": 15}
2024-07-23
T00:26:00.745Z	info	ResourceMetrics #0
Resource SchemaURL: https://opentelemetry.io/schemas/1.25.0
Resource attributes:
     -> service.name: Str(echo)
     -> service.version: Str(HEAD)
     -> telemetry.sdk.language: Str(go)
     -> telemetry.sdk.name: Str(opentelemetry)
     -> telemetry.sdk.version: Str(1.27.0)

Interestingly, it looks like the metric logging races in a slightly inconsistent way. If you call a verb that calls another verb, you would expect just one verb to get bumped to 1 first, and then the other verb. Sometimes, it does behave that way. However, other times, the first time it appears in otel-stream will already be with both verbs. So essentially, there is no guarantee that transient values actually show up in the stream, but the ultimate value will be correct.

This does appear the report the correct count in even error cases. e.g. I tried calling echo before the deploy was done, so that the verb call would fail without calling out to time. It would correctly log a single call in that case. And if you made another successful call afterwards, the metric would correctly increment to include both successful calls.

[wesbillman]

Nice! We can start with this for testing, and move into a dedicated file in the future to keep the controller light :)

[deniseli]

Linking issue: #2194