feat: add support for exporting metrics

Description

Introduces OpenTelemetry 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

Closes #68

Follows the implementations in JS (https://github.com/openfga/js-sdk/pull/117), Python (https://github.com/openfga/python-sdk/pull/107) and Java (https://github.com/openfga/java-sdk/pull/94)

Review Checklist

[x] I have clicked on "allow edits by maintainers".
[ ] I have added documentation for new/changed functionality in this PR or in a PR to 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

Files with missing lines	Patch %	Lines
src/OpenFga.Sdk/Telemetry/Attributes.cs	65.18%	47 Missing and 8 partials :warning:
src/OpenFga.Sdk/Configuration/TelemetryConfig.cs	67.30%	17 Missing :warning:
src/OpenFga.Sdk/Telemetry/Metrics.cs	72.72%	5 Missing and 7 partials :warning:
src/OpenFga.Sdk/ApiClient/OAuth2Client.cs	80.00%	2 Missing and 6 partials :warning:
src/OpenFga.Sdk/Telemetry/Meters.cs	33.33%	8 Missing :warning:
src/OpenFga.Sdk/ApiClient/RequestBuilder.cs	65.00%	3 Missing and 4 partials :warning:
src/OpenFga.Sdk/Telemetry/Histograms.cs	61.11%	6 Missing and 1 partial :warning:
src/OpenFga.Sdk/ApiClient/BaseClient.cs	81.25%	1 Missing and 2 partials :warning:
src/OpenFga.Sdk/ApiClient/ApiClient.cs	93.93%	1 Missing and 1 partial :warning:
src/OpenFga.Sdk/Configuration/Configuration.cs	83.33%	0 Missing and 2 partials :warning:
... and 1 more

openfga / dotnet-sdk