## Motivation and Context
Version `User-Agent` header string and begin tracking business metrics
in that header
## Description
This PR versions `User-Agent` string and the version is set to `2.1`.
Furthermore, we track business metrics for SDK features in `User-Agent`
header. Specifically, we now track the following metrics in the
User-Agent header:
- RPC V2 CBOR (M)
- Gzip request compression (L)
- paginator (C)
- waiter (B)
Each letter corresponds to a metric value defined [in the
specification](3f3c874c9f/aws/rust-runtime/aws-runtime/src/user_agent/metrics.rs (L207-L226))).
#### Overall implementation strategy ####
Since business metrics is an AWS-specific concept, the
`aws-smithy-runtime` crate cannot directly reference it or call
`AwsUserAgent::add_business_metric`. Instead, the crate tracks Smithy
SDK features using the config bag with the `StoreAppend` mode. During
the execution of `UserAgentInterceptor::modify_before_signing`, this
method retrieves the SDK features from the config bag and converts them
into business metrics. This implies that any SDK features—whether
specific to Smithy or AWS—that we intend to track must be added to the
config bag prior to the invocation of the `modify_before_signing`
method.
## Testing
- Added a test-only utility function,
`assert_ua_contains_metric_values`, in the `aws-runtime` crate to verify
the presence of metric values in the `User-Agent` string. Since the
position of metric values in the `business-metrics` string may change as
new metrics are introduced (e.g., previously `m/A` but now `m/C,A,B`),
it is essential that this function accounts for potential variations and
does not rely solely on substring matching.
- Added unit and integration tests to verify tracking of the business
metrics introduced in this PR: RPC V2 CBOR, Gzip request compression,
paginator, and waiter.
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