Add timing information for acquiring an existing connection

[jrhee17]

Motivation:

It has been pointed out that pendingAcquisitionDuration denotes only the duration during which a connection is newly acquired. I propose that we add an existingAcquisitionDuration to clearly differentiate between

1. Waiting for a connection that is pending in completing
2. Waiting for a connection that already exists

Modifications:

• Add ClientConnectionTimings#existingAcquisitionStartTimeMicros, ClientConnectionTimings#existingAcquisitionStartTimeMillis, ClientConnectionTimings#existingAcquisitionDurationNanos
• Add existing.acquisition.duration to RequestMetricSupport
  • Note that although the original issue suggested we migrate all metrics related to request timing, I don't think this has to be necessarily done in this PR
• Rename brave spans annotations "connection-reuse.start", "connection-reuse.end" -> "connection-pending.start", "connection-pending.end"
• Add brave span annotations "connection-existing.start", "connection-existing.end"

Result:

• Closes #4886
• Describe the consequences that a user will face after this PR is merged.

[trustin]

@jrhee17 Please let us know once it's ready!

[jrhee17]

Looking back into this, I'm not sure if there is really much value in adding a meter for this particular scenario. Let me close this PR and revisit later