[processor/spanmetrics] Span metrics produces histograms with incorrect bucket counts

[jack-berg]

Describe the bug
Given a set of N bucket boundaries, the spanmetrics processor produces N bucket counts, where it should produce N+1.

Steps to reproduce
Send some spans through the processor and export the data with the logging exporter. Notice data that looks like:

collector-collector-1  | Metric #5
collector-collector-1  | Descriptor:
collector-collector-1  |      -> Name: latency
collector-collector-1  |      -> Description: 
collector-collector-1  |      -> Unit: 
collector-collector-1  |      -> DataType: Histogram
collector-collector-1  |      -> AggregationTemporality: AGGREGATION_TEMPORALITY_DELTA
collector-collector-1  | HistogramDataPoints #0
collector-collector-1  | Data point attributes:
collector-collector-1  |      -> service.name: STRING(foo)
collector-collector-1  |      -> operation: STRING(ping)
collector-collector-1  |      -> span.kind: STRING(SPAN_KIND_INTERNAL)
collector-collector-1  |      -> status.code: STRING(STATUS_CODE_UNSET)
collector-collector-1  |      -> http.method: STRING(GET)
collector-collector-1  | StartTimestamp: 2022-06-28 17:34:55.393150425 +0000 UTC
collector-collector-1  | Timestamp: 2022-06-28 17:35:00.868986261 +0000 UTC
collector-collector-1  | Count: 4
collector-collector-1  | Sum: 347.312458
collector-collector-1  | ExplicitBounds #0: 0.100000
collector-collector-1  | ExplicitBounds #1: 1.000000
collector-collector-1  | ExplicitBounds #2: 2.000000
collector-collector-1  | ExplicitBounds #3: 6.000000
collector-collector-1  | ExplicitBounds #4: 10.000000
collector-collector-1  | ExplicitBounds #5: 100.000000
collector-collector-1  | ExplicitBounds #6: 250.000000
collector-collector-1  | ExplicitBounds #7: 9223372036854.775391
collector-collector-1  | Buckets #0, Count: 0
collector-collector-1  | Buckets #1, Count: 0
collector-collector-1  | Buckets #2, Count: 0
collector-collector-1  | Buckets #3, Count: 0
collector-collector-1  | Buckets #4, Count: 0
collector-collector-1  | Buckets #5, Count: 2
collector-collector-1  | Buckets #6, Count: 2
collector-collector-1  | Buckets #7, Count: 0

What did you expect to see?
8 buckets counts for the 7 bucket boundaries, as described in the histogram proto.

What did you see instead?
7 bucket counts for 7 bucket boundaries.

What version did you use?
Head

Additional context
Perhaps the root cause of #11782.

[TylerHelmuth]

pinging @albertteoh as code owner

[albertteoh]

Thanks for reporting this @jack-berg (and for the ping @TylerHelmuth 😄 ).

I've created the following PR to address this bug. I hope I've understood the bug correctly; please let me know if not: #11884.