[Remote Vector Index Build] fix: end remote build metrics before falling back to CPU, log exceptions

[owenhalpert]

Description

Because it is in the finally block, the call to endRemoteBuildMetrics is happening after the fallback strategy so the failure time is inaccurate.

Additionally, Exceptions in the remote build process are caught and rethrown to let each step give a failure message:

k-NN/src/main/java/org/opensearch/knn/index/codec/nativeindex/remote/RemoteIndexBuildStrategy.java

Lines 251 to 253 in 76d6662

	} catch (Exception e) {
	log.debug("Repository read failed for vector field [{}]", indexInfo.getFieldName());
	throw e;

We should add the exception itself to these debug logs to add more insight into why the failure occurred (for example, adding this to the submitBuild step will give the HTTP status code instead of just saying "Submit vector build failed". This is already the case for awaitVectorBuild.

Related Issues

N/A

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[navneet1v]

Can we directly raise pr against 3.0 branch since all CIs are failing