[Feature Request] Support metrics for otelgin

[anoopknayak]

Problem Statement

Currently otelgin only provides ability to trace events. I want to propose the ability to monitor gin endpoints using OTEL.

Proposed Solution

The idea is to inject the various instruments within the current Middleware for tracing. We can create a single Meter for all gin endpoint related instrumentation. We want to measure the 4 golden signals and create instruments for measuring all the 4 signals. It would look something like follows:

Metric Name	Type of Instrument	Golden Signal Measured
http.server.request.duration	Int64Histogram	Latencu, Errors (through attributes)
http.server.active_requests	Int64UpDownCounter	Traffic, Errors (through attributes)
http.server.request.body.size	Int64UpDownCounter	Saturation
http.server.response.body.size	Int64UpDownCounter	Saturation

Alternatives

The alternative solution would be to create a new middleware specifically for metrics which IMO might be waste of resources as we are computing the same things for tracing as well.

Prior Art

N/A

Additional Context

N/A

[anoopknayak]

@hanyuancheung Could you help me with this?

[trajan0x]

@technologize put together a demo here

This package also supports both and might be helpful as a reference

(We also adapted it to a later version of otel here if anyone else stumbling across this issue finds useful synapsecns/sanguine#2807)