feat(metrics): add service operation to metrics label

[dentiny]

Which issue does this PR close?

Closes #7310

Rationale for this change

In production when using opendal, one big pain point is I don't have metrics aggregated by S3 HTTP request type.
In detail, multipart upload contains multiple types of requests: initiation, upload part, and completion; each type of request correspond to different server handling and displays different success rate and latency; current client side metrics doesn't provide us enough insight.

This PR adds certain label for HTTP request metrics.

What changes are included in this PR?

In the attached issue, I follow @Xuanwo's proposal to add an extension, similar to what do we do for operation label.

A few changes:

• I add the HTTP request type to metrics, and support exporter for prometheus/fastmetrics/otelmetrics
• I include the HTTP type in request extension, so it could be picked up at metrics layer

A few considerations

• If certain storage backend doesn't support service operation, "unknown" will be used
• The caveat is, for backends which don't support certain extension, (1) for prometheus exporter, metrics sent will contain larger payload; (2) little impact in terms of storage (time-series DB usually does dedup)
• If the community is ok with the change, I plan to rollout the extension to all storage backends (if applicable)

Are there any user-facing changes?

No

AI Usage Statement

I used opus 4.6 to make most of the code change, and I'm responsible for code review/quality.