[docdb] Reduce memory footprint of the per tablet Histograms

[bmatican]

@amitanandaiyer brought this up as part of the investigation in #6676

We should be able to optimize our Histogram usage, to instead of storing these per tablet, bubble these up per table. We could maybe store references to these at the TSTabletManager level and flow these into the Tablet instances, to keep most of the existing code the same.

Other notes

• we are already doing per-table aggregations in Prometheus, so this would only affect the per-tablet versions in the JSON /metrics, which is less relevant anyway
• we already reset the histograms whenever we query from Prometheus
• need to do something special for GC, in case a table is fully deleted / a TS no longer serves any tablets of a particular table, but that should be easy to detect..

[amitanandaiyer]

Images posted on the commit essentially verify that the TabletMetrics in-use go down from 512 (i.e. # of tablets) at each call stack to 1 (# of tables created) per call stack.

Before commit a4eb8ae :

07:40 ~/code/yugabyte-db [more_traces] $ grep -B 5 "yb::tablet::Table.*Metrics::Table.*Metrics" ~/Downloads/512_tablets.in_use_bytes.html | grep -A 1 -B 5 "yb::HistogramPrototype::Instantiate()" | grep -A 5 -B 2 "yb::HdrHistogram::Init()"
<tr>
<td>512</td><td>10485760</td><td>20480.00</td><td>512</td><td>10485760</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>512</td><td>10485760</td><td>20480.00</td><td>512</td><td>10485760</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>512</td><td>10485760</td><td>20480.00</td><td>512</td><td>10485760</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>512</td><td>10485760</td><td>20480.00</td><td>512</td><td>10485760</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>512</td><td>10485760</td><td>20480.00</td><td>512</td><td>10485760</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--

After commit a4eb8ae

07:39 ~/code/yugabyte-db [more_traces] $ grep -B 5 "yb::tablet::Table.*Metrics::Table.*Metrics" 512_tablets_with_fix.in_use_bytes.html | grep -A 1 -B 5 "yb::HistogramPrototype::Instantiate()" | grep -A 5 -B 2 "yb::HdrHistogram::Init()"
--
<tr>
<td>1</td><td>20480</td><td>20480.00</td><td>1</td><td>20480</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>1</td><td>20480</td><td>20480.00</td><td>1</td><td>20480</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>1</td><td>20480</td><td>20480.00</td><td>1</td><td>20480</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>1</td><td>20480</td><td>20480.00</td><td>1</td><td>20480</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
--
--
--
<tr>
<td>1</td><td>20480</td><td>20480.00</td><td>1</td><td>20480</td><td>20480.00</td><td><pre>yb::HdrHistogram::Init()
yb::HdrHistogram::HdrHistogram()
yb::Histogram::Histogram()
yb::HistogramPrototype::Instantiate()
yb::tablet::TabletMetrics::TabletMetrics()
--
07:40 ~/code/yugabyte-db [more_traces]