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## Description

Introduces [OpenTelemetry](https://opentelemetry.io/) metrics reporting
into the SDK for specific actions. We're intentionally starting with
only a few metric events that we can then grow over time.

We don't have any formal documentation for this currently. We'll write
this as we roll it out across all SDKs, so here is a summary of the
events and the associated attributes:

## Metrics

### Supported Metrics

| Metric Name | Type | Enabled by Default | Description |

|---------------------------------|-----------|--------------------|--------------------------------------------------------------------------------------|
| `fga-client.request.duration` | Histogram | Yes | The total request
time for FGA requests |
| `fga-client.query.duration` | Histogram | Yes | The amount of time the
FGA server took to internally process nd evaluate the request |
|` fga-client.credentials.request`| Counter | Yes | The total number of
times a new token was requested when using ClientCredentials |
| `fga-client.request.count` | Counter | No | The total number of
requests made to the FGA server |

### Supported attributes

| Attribute Name | Type | Enabled by Default | Description |

|--------------------------------|----------|--------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `fga-client.response.model_id` | `string` | Yes | The authorization
model ID that the FGA server used |
| `fga-client.request.method` | `string` | Yes | The FGA method/action
that was performed (e.g. `Check`, `ListObjects`, ...) in TitleCase |
| `fga-client.request.store_id` | `string` | Yes | The store ID that was
sent as part of the request |
| `fga-client.request.model_id` | `string` | Yes | The authorization
model ID that was sent as part of the request, if any |
| `fga-client.request.client_id` | `string` | Yes | The client ID
associated with the request, if any |
| `fga-client.user` | `string` | No | The user that is associated with
the action of the request for check and list objects |
| `http.request.resend_count` | `int` | Yes | The number of retries
attempted (Only sent if the request was retried. Count of `1` means the
request was retried once in addition to the original request) |
| `http.response.status_code` | `int` | Yes | The status code of the
response |
| `http.request.method` | `string` | No | The HTTP method for the
request |
| `http.host` | `string` | Yes | Host identifier of the origin the
request was sent to |
| `url.scheme` | `string` | No | HTTP Scheme of the request
(`http`/`https`) |
| `url.full` | `string` | No | Full URL of the request |
| `user_agent.original` | `string` | Yes | User Agent used in the query
|

## Testing

Spin up Prometheus, build the project and then run the
OpenTelemetryExample app.

You should see metrics in Prometheus such as:
```
fga_client_query_duration_bucket{exported_instance="28790af8-2116-4e47-baaf-319f9f23f518", exported_job=""openfga-otel-dotnet-example"", fga_client_request_client_id="3DSvMhkJwjoSBMTliPEdPjybbVewfVes", fga_client_request_method="Check", fga_client_request_model_id="01J41WCA3KBA9SP2M350FPDEJ0", fga_client_request_store_id="01J40DW2J9NNYNNVEC430S3Z97", fga_client_response_model_id="01J41WCA3KBA9SP2M350FPDEJ0", fga_client_user="user:anne", http_client_request_duration="40", http_host="api.us1.fga.dev", http_request_method="POST", http_response_status_code="200", http_server_request_duration="2", instance="otel-collector:8889", job="otel-collector", label1="value1", le="5", url_full="https://api.us1.fga.dev/stores/01J40DW2J9NNYNNVEC430S3Z97/check", url_scheme="https", user_agent_original="openfga-sdk dotnet/0.4.0"}
---
fga_client_query_duration_bucket{exported_instance="fbee572d-0bff-4957-95a2-c532fe7312c1", exported_job=""openfga-otel-dotnet-example"", fga_client_request_client_id="3DSvMhkJwjoSBMTliPEdPjybbVewfVes", fga_client_request_method="WriteAuthorizationModel", fga_client_request_store_id="01J40DW2J9NNYNNVEC430S3Z97", http_client_request_duration="139", http_host="api.us1.fga.dev", http_request_method="POST", http_request_resend_count="1", http_response_status_code="201", http_server_request_duration="69", instance="otel-collector:8889", job="otel-collector", label1="value1", le="75", url_full="https://api.us1.fga.dev/stores/01J40DW2J9NNYNNVEC430S3Z97/authorization-models", url_scheme="https", user_agent_original="openfga-sdk dotnet/0.4.0"}
---
fga_client_query_duration_bucket{exported_instance="fbee572d-0bff-4957-95a2-c532fe7312c1", exported_job=""openfga-otel-dotnet-example"", fga_client_request_client_id="3DSvMhkJwjoSBMTliPEdPjybbVewfVes", fga_client_request_method="Write", fga_client_request_model_id="01J41WCA3KBA9SP2M350FPDEJ0", fga_client_request_store_id="01J40DW2J9NNYNNVEC430S3Z97", fga_client_response_model_id="01J41WCA3KBA9SP2M350FPDEJ0", http_client_request_duration="68", http_host="api.us1.fga.dev", http_request_method="POST", http_request_resend_count="1", http_response_status_code="200", http_server_request_duration="30", instance="otel-collector:8889", job="otel-collector", label1="value1", le="5000", url_full="https://api.us1.fga.dev/stores/01J40DW2J9NNYNNVEC430S3Z97/write", url_scheme="https", user_agent_original="openfga-sdk dotnet/0.4.0"}
```

Also for client credential exchange, you should see metrics such as:
```
{__name__="fga_client_credentials_request", exported_instance="28790af8-2116-4e47-baaf-319f9f23f518", exported_job=""openfga-otel-dotnet-example"", fga_client_request_client_id="3DSvMhkJwjoSBMTliPEdPjybbVewfVes", fga_client_request_method="ClientCredentialsExchange", http_client_request_duration="1461", http_host="fga.us.auth0.com", http_request_method="POST", http_response_status_code="200", instance="otel-collector:8889", job="otel-collector", label1="value1", url_full="https://fga.us.auth0.com/oauth/token", url_scheme="https", user_agent_original="openfga-sdk dotnet/0.4.0"}
```

## References
<!-- Provide a list of any applicable references here (GitHub Issue,
[OpenFGA RFC](https://github.com/openfga/rfcs), other PRs, etc..) -->

Closes #68

Follows the implementations in JS
(openfga/js-sdk#117), Python
(openfga/python-sdk#107) and Java
(openfga/java-sdk#94)

## Review Checklist
- [x] I have clicked on ["allow edits by
maintainers"](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork).
- [ ] I have added documentation for new/changed functionality in this
PR or in a PR to [openfga.dev](https://github.com/openfga/openfga.dev)
[Provide a link to any relevant PRs in the references section above]
- [x] The correct base branch is being used, if not `main`
- [ ] I have added tests to validate that the change in functionality is
working as expected